What is the basis for believing that 40 lashes would be deadly (especially in Roman law or jurisprudence)?
This is a false assumption!
For one thing, the Romans were not limited in the number of strokes they could mete out.
True some died under the sentence of flagellation, but historians generally do not give the number of strokes applied.
In the Roman Empire, flagellation was often used as a prelude to crucifixion, and in this context is sometimes referred to as scourging. Most famously according to the gospel accounts, this occurred prior to the crucifixion of Jesus Christ.
Whips with small pieces of metal or bone at the tips were commonly used. Such a device could easily cause disfigurement and serious trauma, such as ripping pieces of flesh from the body or loss of an eye. In addition to causing severe pain, the victim would approach a state of hypovolemic shock due to loss of blood.
The Romans reserved this treatment for non-citizens, as stated in the lex Porcia and lex Sempronia, dating from 195 and 123 BC. The poet Horace refers to the horribile flagellum (horrible whip) in his Satires. Typically, the one to be punished was stripped naked and bound to a low pillar so that he could bend over it, or chained to an upright pillar so as to be stretched out. Two lictors (some reports indicate scourgings with four or six lictors) alternated blows from the bare shoulders down the body to the soles of the feet. There was no limit to the number of blows inflicted—this was left to the lictors to decide, though they were normally not supposed to kill the victim. Nonetheless, Livy, Suetonius and Josephus report cases of flagellation where victims died while still bound to the post. Flagellation was referred to as "half death" by some authors, as many victims died shortly thereafter. Cicero reports in In Verrem, "pro mortuo sublatus brevi postea mortuus" ("taken away for a dead man, shortly thereafter he was dead"). - Flagellation (Wikipedia)
The floggings used by Romans were by far more sever than the ones used by the Jewish people in ancient times. It wasn't uncommon for the victims of the Roman scourge to die from the ensuing blood loss and/or shock (See: here). The Centurion in charge would order the "lictors" to halt the flogging when the criminal was near death.
Scourging, called verberatio by the Romans, was possibly the worst kind of flogging administered by ancient courts. While the Jews administered whippings in the synagogues for certain offenses, these were mild in comparison to scourging. Scourging was not normally a form of execution, but it certainly was brutal enough to be fatal in many cases. A person certainly could be beaten to death by the scourge if that was desired. Its purpose was not only to cause great pain, but to humiliate as well. To scourge a man was to beat him worse than one would beat a stupid animal. It was belittling, debasing, and demeaning. It was considered such a degrading form of punishment that, according to the Porcian (248 B.C.) and Sempronian (123 B.C.) laws, Roman citizens were exempt from it. It was, therefore, the punishment appropriate only for slaves and non-Romans, those who were viewed as the lesser elements in Roman society. To make it as humiliating as possible, scourging was carried out in public.
The instrument used to deliver this form of punishment was called in Latin a flagellum or a flagrum. This was much different from the bull whip that is more common in our culture. It was instead more like the old British cat o’ nine tails, except that the flagellum was not designed merely to bruise or leave welts on the victim. The flagellum was a whip with several (at least three) thongs or strands, each perhaps as much as three feet long, and the strands were weighted with lead balls or pieces of bone. This instrument was designed to lacerate. The weighed thongs struck the skin so violently that it broke open. The church historian Eusebius of Caesarea recounts with vivid, horrible detail a scene of scourging. He says, “For they say that the bystanders were struck with amazement when they saw them lacerated with scourges even to the innermost veins and arteries, so that the hidden inward parts of the body, both their bowels and their members, were exposed to view” (Ecclesiastical History, Book 4, chap. 15).
The victim of a scourging was bound to a post or frame, stripped of his clothing, and beaten with the flagellum from the shoulders to the loins. The beating left the victim bloody and weak, in unimaginable pain, and near the point of death. It is no doubt that weakness from his scourging was largely the reason Jesus was unable to carry his cross all the way to Golgotha (Matt. 27:32 and parallels).
As noted above, the beating administered by synagogues was not nearly as drastic as a Roman scourging. First, the instrument used in the synagogues was a lighter whip and was not weighted with metal or bone. Second, according to the tradition recorded in the Mishnah (tractate Makkot), the judges would determine if the victim could survive the full measure of the beating required by the law (forty lashes). If he could not, the number of lashes was reduced. Third, the Law of Moses limited whippings to forty lashes (Deut. 25:3), which was a provision to prevent excessive humiliation. The Jews usually stopped at thirty-nine (lest they counted wrong and violated the law by giving more than forty; cf. Paul’s reference to “thirty-nine stripes” in 2 Cor. 11:24). Scourging, however, was much more traumatic, even to the point of being fatal. The flagellum was a much more torturous instrument, the lashes were delivered without any compassion or consideration for the victim’s health, and Roman law imposed no limit to the number of lashes inflicted at scourging. Roman law mandated scourging as part of capital sentences, but this probably had the effect of shortening the victim’s agony once on the cross. The victim would have been so weak from blood loss and pain that he would die more quickly than if he had not been scourged. This seems to have been the case with Jesus (although the scourging was probably not the only thing that caused him to die relatively quickly). - The Scourging of Jesus
Taking the above into consideration, we can see that Hebrew Law something quite different in how they dealt with criminal behaviour.
Not only had the ancient Hebrews had a mild form of whipping people, they also limited the number of strokes to 40. And later it was reduced to 39 in order to avoid giving more than 40 lashes by accident.
The Romans used a number of differ whips, when they chastised someone. Generally speaking the type of instrument used depended on the offence committed.
Judges among the Romans, as has been just now mentioned, used a great variety of instruments for inflicting the punishment of whipping. Some consisted of a flat strap of leather, and were called Ferulae; and to be lashed with these Ferulæ, was considered as the mildest degree of punishment. Others were made of a number of cords of twisted parchment, and were called Scuticæ. These Scuticæ were considered as being a degree higher in point of severity than the Ferulæ, but were much inferior in that respect, to that kind of scourge which was called Flagellum, and sometimes the Terrible Flagellum, which was made of thongs of ox-leather, the same as carmen used for their horses. We find in the third satire of the first book of Horace, a clear and pretty singular account of the gradation in point of severity that obtained between the above-mentioned instruments of whipping. In this satire, Horace lays down 11 the rules which he thinks a judge ought to follow in the discharge of his office; and he addresses himself, somewhat ironically, to certain persons who, adopting the principles of the Stoics, affected much severity in their opinions, and pretended that all crimes whatever being equal, ought to be punished in the same manner. “Make such a rule of conduct to yourself (says Horace) that you may always proportion the chastisement you inflict to the magnitude of the offence; and when the offender only deserves to be chastised with the whip of twisted parchment, do not expose him to the lash of the horrid leather scourge, for that you should only inflict the punishment of the flat strap on him who deserves a more severe lashing, is what I am by no means afraid of.” - History of Flagellation
Flogging for the Jews was a discipline measures in order to correct their fellow Israelite. One was encumbered to discipline their fellow man and not to degrade him into an animal, to maim him or to take his life! They probably used some form of reed, just as the ancient Chinese used bamboo for the purpose of flagellation.
The Jews never used the Roman Flagellum as a instrument of torture, for in was without doubt reserved for extreme cases of criminal discipline, usually involving execution.
Forty stripes he may give him, and not exceed: lest, if he should exceed, and beat him above these with many stripes, then thy brother should seem vile unto thee. - Deuteronomy 25:3 (KJV)
Although Scriptures tell us that Moses limited the Number of lashes to 40. The number often was reduced to 39.
According to the Torah (Deuteronomy 25:1-3) and Rabbinic law lashes may be given for offenses that do not merit capital punishment, and may not exceed 40. However, in the absence of a Sanhedrin, corporal punishment is not practiced in Jewish law. Halakha specifies the lashes must be given in sets of three, so the total number cannot exceed 39. Also, the person whipped is first judged whether they can withstand the punishment, if not, the number of whips is decreased. Jewish law limited flagellation to forty strokes, and in practice delivered thirty-nine, so as to avoid any possibility of breaking this law due to a miscount.
In Talmudic Law the number was reduced by one in case of miscounting.
Talmudic law not only made detailed provision for the manner in which floggings were to be carried out, but also altered the concept of the biblical punishment; the maximum of 40 lashes was reduced to 39 (Mak. 22a), so as to avoid the danger of exceeding 40 even by mistake; and the offenses which carried the punishment of flogging were exactly defined, depriving it of its character as a residuary and omnibus punishment. The number of 39 lashes became the standard rather than the maximum number; but in order to prevent death by flogging – which would amount to a violation of the biblical injunction of "not more" than flogging – the person to be flogged was first physically examined in order to determine the number of lashes that could safely be administered to him (Mak. 3:11). Where, as a result of such examination, less than 39 lashes were administered, and it then turned out that the offender could well bear more, the previous estimate would be allowed to stand and the offender discharged (Maim. Yad, Sanhedrin 17:2). But the offender would also be discharged where physical symptoms manifested themselves during the course of the flogging, so that he would not be able to stand any more lashes, even though on previous examination he had been found fit to stand more (ibid. 17:5). It also happened that as a result of such examination, floggings were postponed for another day or later, until the offender was fit to undergo them (ibid. 17:3).
Written evidence from the time of Jesus reveals that torture was not only carried out but actually regulated under the Roman state. A stone inscription found in the modern Italian town of Pozzuoli (ancient Puteoli), dating to the first century C.E., details regulations for the hiring of people to torture or execute slaves, whether by court order or in response to an owner’s request:
[Members of t]he workforce which shall be provided for ... inflicting punishment ... None of them is to be over fifty years of age or under twenty, nor have any sores, be one-eyed, maimed, lame, blind, or branded. The contractor is to have no fewer than thirty-two operatives.
If anyone wishes to have a slave – male or female – punished privately, he who wishes to have the punishment inflicted shall do so as follows. If he wants to put the slave on the cross or fork, the contractor must supply the posts, chains, ropes for flog- gers, and the floggers themselves. ... The magistrate shall give orders for such punish- ments as he exacts in his public capacity, and when orders are given (the contractor) is to be ready to exact the punishment. He is to set up crosses and supply without charge nails, pitch, wax, tapers, and anything else that is necessary for this in order to deal with the condemned man ...(The Roman World: A Sourcebook, David Cherry, editor, Blackwell Publishers 2001, pp. 26-27; text translation from J. F. Gardiner and T. Wiedemann, The Roman Household: A Sourcebook, London 1991, pp. 24-26).
The linked video in the article states that according to the Shroud of Turin, the man in the image received over 120 lashes.
The following article(s) May be of interest: