someone suggested he could be Leonid Kovalev but according to some google search, somone already mentioned on the chat a while ago that this was unlikely
@WhitAngl Since he explicitly wanted his contributions to be anonymized (which seems to be the only real reason for deleting his account), I don't see how anyone could fault you for citing him as simply as "an anonymous user of the website math.stackexchange.com".
I would treat it as a user who chooses to not give his real name unless you can think of something more clever. Perhaps use a mixture of this and whatever comes up from the \cite functionality
@Henning he might want to remain anonymous from math.SE but it doesn't mean he doesn't want to remain anonymous for everything (which includes publications) ;)
@WhitAngl But that particular thing he wrote on MSE, that he does want to be anonymous. If you learned his identity and publicly revealed the author of the person who helped you on MSE, his anonymity here would be blown.
@Henning this is the question I'd like to acknowledge: math.stackexchange.com/questions/264405/… which is authored by me. Of course, his crazy ex gf could also have written the question and created a fake account years ago to track him down ;)
@WhitAngl That's three months ago. Some people burn through relationships quickly. Perhaps not even an ex -- what if you communicated in person after that exchange and you developed a crazy stalkerish obsession with her that she had to change her phone number and burn her account on the site to escape?
@Henning hehe :) At best I can send a version of the paper lacking his name. The paper has been accepted last week (that's quite fast : this is a conference in computer science, and dates can be checked). Contrary to mathematics, I guess in such circumstances only clues can be given rather than proofs ;)
@mixedmath My point is just that when people deliberately and explicitly decide to disappear, other users who might know who they are can be argued to have a moral duty not to reveal this information to random people who ask for it in chat.
@WhitAngl Do note that I'm of course not accusing you of anything that fantastic.
@HenningMakholm Indeed, I largely agree. I think part of the reason I would not be against contacting him is because I do not think the main reason he left the site was for anonymity. Even then, I'd send him an email asking permission before having WhitAngl contact him