Lichess is also open source. They are transparent with their finances and have taken on a developer who is upgrading their app(s).
I say plural as they are keeping the old one with minor fixes but building a new one from the ground up .
The new app can also let you play puzzles while offline and well as play a bot offline.
Major variants are supported like 960, triple chess, atomic, and antichess.
And I think they're also big on not having any trackers on the site too.
Lately I've started using fediverse presence as an early indicator to decide whether I trust an organization or company. Proton leaving because they couldn't stand the heat after coming out as terrible kinda proved my point. Mozilla shutting down their instance was a good red flag.
In part it has to do with matching overly consistently with the moves engines would pick. Even the best players in the world perform worse than modern engines, not to mention you'd expect human players to vary at least sometimes by personal preference among roughly equivalent options.
There are obvious patterns, like the time to make a move.
Additionally, chess engines make moves that don't really make sense to humans because they are making considerations hundreds of moves out, so if a player, through multiple games, consistently makes moves that are the best or close to the best engine moves, but don't really make sense, that pattern can be analyzed.
One fluke of a brilliant move that obviously doesn't mean anything. But if you are consistently playing 1000 moves ahead, that's obviously impossible.
Combine that with user reports to see what players to deeply analyze it can work well.
It helps to consider that computers are significantly better than humans at chess, so much so that they would say the best technical move is one that looks like a blunder to most players, including professionals.
Not a player either. My understanding is that there are a couple giveaways.
AI play very differently than people. The thinking* for AI is shortest term and it's my understanding that it's obvious to people that play. See below.
It's possible to look up optimal* moves based on the board state. I would imagine that they'll keep an eye on your browser/system to keep an eye out for such tools.
Lichess has a vastly superior puzzle engine, it takes moments from real games where the player have made a mistake, catalogues it, ranks it and you can even check out the real game if you want.
In the chess streamer world, they all play chess.com because they get paid too. And there are gamification features like novelty 'bots' and daily rewards.
Lichess is much more serious and is solely focused on the chess experience