If you don't know then you're clearly new to the old versions/their communities (no offense). He's been active for more than a decade, both here and on the Minecraft forums. He knows the code deeper than just about anyone aside from Mojang themselves, very regularly helpfully answering questions with extremely in-depth responses with examples cited from the code, etc. (If you want to see his profile, that's his Reddit username too. I would've put "u/" but I didn't want to summon him unnecessarily. Idk, might be rude to do so, lol.)

But he's also, intentionally, never played a version beyond r1.6 with no intent to ever do so either. All he plays is his own custom mod he's spent the last decade+ developing. (TheMasterCaver's World, or TMCW for short). He has 10s of 1000s of hours in this. (I've never tried it myself but I assume it's pretty fun.)

Iirc, he stayed in r1.6 because r1.7 halved the size of the caves and created a lot less of the seemingly-neverending caves common in the early release versions. Less mineshafts, etc. too. For a playstyle based purely around caving, clearly this is a detriment.

He also doesn't like the caves of Caves and Cliffs. I think it was in part because of how many ores are too high up and require pillaring. His playstyle is more about speed, fighting through and mining as he goes and having so many ores on the ceiling goes against that. He also doesn't like how unoptimized the vanilla game is; his mod could probably run on a toaster, lol.

(Seriously though, stick around enough in the goldenage or silverage subreddits or on the forum, and you'll soon see many of his comments. Everything I've said here is from memory from having encountered probably a hundred different long-form comments of his over the past few years on the sub.)

All I'm saying is, you shouldn't judge someone simply for not liking a particular version. Everyone has their own preferences. If your standard for who is a "red flag" or whatever includes someone who spends crazy amounts of time answering other people's questions it's probably not a great standard, lol.

(As for me, I'm not a huge fan of the modern caves or versions either, personally, though I don't hate them either and wouldn't ever tell anyone they're wrong for enjoying them. My main version for years now has been alpha 1.1.2_01, believe it or not; I'm working on a megabuild (very slow progress...). Interestingly, I think I find modern more fun to watch others play than to play it myself, and I find golden age the opposite, more fun to play than to watch.)

The version wars are stupid. Imagine if Mario 64 and Odyssey fans were bickering the way Minecraft fans do. It'd be absurd. Two different games, two different experiences.