Further to the excellent answer provided by Agarza explaining the changes that now take effect regarding the treatment of disfellowshipped Witnesses, this article published August 2024, and to be studied in Kingdom Halls in October this year, specifically deals with your main question. Here is a summary of some of the main points.
Paragraphs 10 and 11 discuss how the elders may reinstate Witnesses who were disfellowshipped in the past but who no longer practice the sin for which they were removed from the congregation and who wish to return.
When a sinner manifests repentance and abandons his wrong course, he can be reinstated without delay.—2 Cor. 2:6-8.
Caution must be exercised if a person was guilty of child abuse or apostasy or if he schemed to end a marriage. Apostasy is when a baptised Witnesses openly disagrees with the teachings of the organisation and who speaks out against them. Fidelity within a marriage and morality are also viewed (rightly) as essential. True repentance and evidence that a wrong course has been abandoned must be manifest before the elders could reinstate such persons.
Perhaps the most significant change is to be found in paragraph 14:
14 Does what we have considered mean that we would completely ignore a person who has been removed from the congregation? Not necessarily. Certainly, we would not socialize with him. But Christians can use their Bible-trained conscience in deciding whether to invite a person who was removed from the congregation—perhaps a relative or someone they were close to previously—to attend a congregation meeting. What if he attends? In the past, we would not greet such a person. Here again, each Christian needs to use his Bible-trained conscience in this matter. Some may feel comfortable with greeting or welcoming the person to the meeting. However, we would not have an extended conversation or socialize with the individual.
Christians can use their Bible-trained conscience when deciding whether to invite a person who has been removed from the congregation to a meeting or to welcome the individual to the meeting with a simple greeting.
Previously, any member who was disfellowshipped for immorality, apostasy or being found guilty of child abuse, would be totally shunned, not only by the congregation but by their own family. This change shows a real softening of attitudes from the leaders.
The exception to this relaxation of the rules does not apply to apostates and those who actively promote wrong conduct:
Therefore, if a person is actively promoting apostate teachings or other wrongdoing, the elders would not arrange to visit him. Of course, there is hope that he will come to his senses. Until that happens, though, we would neither greet such a person nor invite him to attend a congregation meeting.
We will no longer refer to such ones as being disfellowshipped. In harmony with Paul’s words recorded at 1 Corinthians 5:13, we will now refer to them as being removed from the congregation.
According to the Bible, an unforgivable sin is not a particular type of sin but a sin that is committed with a hardened attitude that is permanently opposed to God. Whether a person has committed such a sin is not for us to judge.
Apart from the Watchtower Study Article 35 (above) I found an older official source from Insight on the Scriptures This answers your question about the previous teachings:
Some of the offenses that could merit disfellowshipping from the Christian congregation are fornication, adultery, homosexuality, greed, extortion, thievery, lying, drunkenness, reviling, spiritism, murder, idolatry, apostasy, and the causing of divisions in the congregation. (1Co 5:9-13; 6:9, 10; Tit 3:10, 11; Re 21:8) Mercifully, one promoting a sect is warned a first and a second time before such disfellowshipping action is taken against him. In the Christian congregation, the principle enunciated in the Law applies, namely, that two or three witnesses must establish evidence against the accused one. (1Ti 5:19) Those who have been convicted of a practice of sin are reproved Scripturally before the “onlookers,” for example, those who testified concerning the sinful conduct, so that they too may all have a healthy fear of such sin.—1Ti 5:20; see REPROOF.
The Christian congregation is also admonished by Scripture to stop socializing with those who are disorderly and not walking correctly but who are not deemed deserving of complete expulsion. Paul wrote the Thessalonian congregation concerning such: “Stop associating with him, that he may become ashamed. And yet do not be considering him as an enemy, but continue admonishing him as a brother.”—2Th 3:6, 11, 13-15.
However, regarding any who were Christians but later repudiated the Christian congregation or were expelled from it, the apostle Paul commanded: “Quit mixing in company with” such a one; and the apostle John wrote: “Never receive him into your homes or say a greeting to him.”—1Co 5:11; 2Jo 9, 10.
Those who have been expelled may be received back into the congregation if they manifest sincere repentance. (2Co 2:5-8) This also is a protection to the congregation, preventing it from being overreached by Satan in swinging from condoning wrongdoing to the other extreme, becoming harsh and unforgiving.—2Co 2:10, 11.
Your final question on what may have precipitated the changes, may be answered in this article
There is also the fact that global % growth has diminished over the past decade (from 2.4% in 2011 to 1.3% in 2023) and that in some countries, growth has been negative. In 2020 there was a global decrease of 0.6% and a decrease of 3% in the U.K. alone. I do not wish to speculate or be uncharitable, but I have access to statistics from previous Annual Year Books to verify this.
A less harsh approach to disfellowshipping and shunning is only to be welcomed.
I don't know if this is an official statistic from the 2024 Service Year Report but there has been a 43% increase in "those who returned to Jehovah" which suggests that this less harsh approach is having positive results.
For the latest statistics from the 2024 worldwide service year report, please see the comments (below) from Kristopher.