Update 2020-05-08: Jacy wrote to me to tell me that his full name is "Jacy Reese Anthis", and that he had been using "Jacy Reese" in his public writing. Since getting married he's switched to using his full name exclusively. I looked at Wikipedia, and I noticed that since I wrote this six months ago, other people had continued trying to edit the name: I see a 1/12 edit by Lockedinthebox reverted the next day by Bodole and a 1/19 edit by Lockedinthebox again, again promptly reverted by Bodole. The back-and-forth ends with a 4/29 Bodole edit to put in the full name, citing Jacy's website and Twitter.

Yesterday I noticed that Wikipedia:Jacy Reese doesn't mention anywhere that until recently he went by Jacy Anthis. Thinking this was an oversight, I looked up the relevant section in the Manual of Style and saw:

In some cases, subjects have had their full names changed at some point after birth. In these cases the birth name should be given in the lead as well.

So I edited the page to add it, changing it from:

To:

Then I thought to look at the talk page, to see whether maybe there was a reason it didn't already give a birth name. It turns out there's been extensive discussion on this, as people keep noticing that his birth name is missing, adding it, and then others removing it for not being verifiable. Looking back I see:

• December 2017: article created, with "Jacy Reese".
• August 2018: an anonymous user changes it to "Jacy Reese Anthis, known professionally as Jacy Reese".
• Three days later: reverted by DominikPeters.
• March 2019: user MoonStarDolphin changes it to "Jacy Reese (born Jacy Anthis)".
• Two days later: reverted by Utsill.
• October 2019: I change it to "Jacy Reese (born Jacy Anthis)".
• Six hours later: reverted by Bodole.

The people adding his birth name are doing it because it's "obvious", sometimes giving citations of his earlier use of "Jacy Anthis", and the people reverting it are pointing out that Wikipedia's rules for Biographies of living persons are really very strict. These rules began after the Seigenthaler biography incident where someone put false information in Seigenthaler's wikipedia article, and are a sensible response to how false information about someone on Wikipedia can be very damaging.

After my edit was reverted I looked around to see if I could find better sources for this, and found:

• A 2015 Animal Charity Evaluators Facebook post saying "In our most recent blog post Jacy Anthis summarized his essay on how the Animal Rights movement can use confrontation, consumer action and triggering events to help non-human animals," and linking to a blog post that now has a "Jacy Reese" byline:

  

• His 2015 speech at the EA Global conference which uses the name "Jacy Anthis":

  

• git commits under github.com/jacyreese with "Jacy Anthis" as the author:

  

• The peculiar #metoo story of animal activist Jacy Reese [EDIT 2020-05-08: that link has broken, but this one works], which says "During his sophomore year at Brown, Reese, who then went by his full name, was expelled from the university after being accused of sexual misconduct" and links to a letter he wrote to the school paper. While the HTML version is signed "Jacy '15", the archived print version is signed "Jacy Anthis '15":

  

• The above are sources I found that link the two names, but there's also a pattern where in late 2015 we move from many references to "Jacy Anthis" and none to "Jacy Reese", and then suddenly it's the opposite.

Reading Wikipedia's policies, however, none of these meet their definition of a "reliable source" for this information. Taken together you could argue that they're sufficient evidence, but that would be original research and Wikipedia (reasonably!) does not allow that. A post like this one also wouldn't be considered reliable, since it's self-published. It sounds to me like the various editors who have reverted this change have been correct to do so.

It's interesting this minor fact that, to several people including me, has seemed like an obvious omission, doesn't meet Wikipedia's standards for inclusion. But if Wikipedia had less strict standards it would be very hard to keep out false information.