I was just reading some history on extreme forms of asceticism and monasticism. In passing the subject of Origen it said he had castrated himself. Is this generally considered true by church historians?
This has been debated, so no one knows for sure. The consensus of the scholars seems to be that this was a malicious rumor about him, possibly started by Demetrios, the Bishop of Alexandria, a prosecutor of Origen.
John McGuckin's Westminster Handbook to Origen states that someone who interpreted the Gospels so allegorically would be unlikely to have interpreted Matthew 19:12 so literally and that in Origen's commentary on Matthew, he himself derides the literal interpretation of the eunuch and says only an idiot would consider that. Origen doesn't use the word "idiot," but you can read his derision for the interpretation in his Commentary on Matthew 15.1-5.
The castration story comes from the historian Eusebius of Caesarea. Eusebius, the church historian who lived a generation after Origen, devotes nearly all of Book VI of his Ecclesiastical History to the life of Origen. While some scholars have cast doubt on the reliability of this source due to the author's obvious admiration for his subject and the work's loving hagiographical tone, most do not believe these factors are reason for doubting Eusebius' reports. However, why take Eusebius' account over Origen's own words?
A nice account can be found here: http://houseoftheinklings.blogspot.com/2006/09/on-origen.html
Many historians argue that Eusebius, who lived only a few decades after Origen, would have access to accurate information and no incentive to lie. Others argue that much of what Eusebius says cannot be considered historically reliable, pointing out that Origen's self-castration would conflict with Origen's theology: he interpreted Mat 18:8-9 as a command to separate from people who are causing one to sin, not a command to rid oneself of body parts. Thus it is very unlikely that he took Matthew 19:12 to be about amputation.
ORIGEN ON MATTHEW 18:8-9: "If your hand or foot causes you to sin, cut it off and cast it from you. It is better for you to enter into life lame or maimed, rather than having two hands or two feet, to be cast into the everlasting fire. And if your eye causes you to sin, pluck it out and cast it from you. It is better for you to enter into life with one eye, rather than having two eyes, to be cast into hell fire."