People always claimed this as a data leak vector but I've always been sceptical. Like just writing style and vocabulary is probably extremely shared among too many people to narrow it down much. (How people that you know could have written this reply?) The counter argument is that he had a very specific style in his mail so maybe this is a special case.

If you have a large enough set to test against and a specific person you are looking for, this is totally doable currently.

this is a well-studied field (stylometry). when combining writing styles, vocabulary, posting times, etc. you absolutely can narrow it down to specific people.

even when people deliberately try to feign some aspects (e.g. switching writing styles for different pseudonyms), they will almost always slip up and revert to their most comfortable style over time. which is great, because if they aren't also regularly changing pseudonyms (which are also subject to limited stylometry, so pseudonym creation should be somewhat randomized in name, location, etc.), you only need to catch them slipping once to get the whole history of that pseudonym (and potentially others, once that one is confirmed).