Hexbear started during the 2020 BLM protests, where cops were using social media and internet presence to track down activists. They are still doing this, but with less vigor than when police stations were burning down.
This included things like using tattoos on naked bodies, etsy store receipts etc.
Just before the r/cth ban, there was also a problem with chasers and leering objectification, and steps were taken to reduce thirst-posting and the like.
These things combined means that people don't post selfies or direct identifying information. People post their pets and artwork, but I know I have to make a decision about where and when I post things to make things non-trivial for cops or random chuds. I feel like a unique pet name, breed, and rough region could be enough to track someone down.
Even so, I think I'm bad at it. I feel like if someone knew me and read everything on hexbear they could ID me pretty easily (and I know multiple people in person on hexbear, but we've never exchanged usernames).
Idk if there are any hard and fast rules beyond the selfies and direct ID though. I should burn this account.
Edit: removed reference to masculinity
Electronic Frontier Foundation has a good collection of articles and guides that are not just practical but teach you how to think about your own particular situation: Surveillance Self-Defense
First thing is you need to think about your "threat model". Which means what information are you trying to protect, and why? There is no universally "best" way to do anything. Tactics that might increase security/privacy in one situation might impair it in another. Like if you are trying to avoid getting doxxed by random internet people who take a dislike to you that's one thing, but if you are trying to avoid government (which government(s)?) spying that's another. If you are trying to avoid government spying, is it to avoid legal prosecution, or to avoid covert interference in your organizing (ala COINTELPRO)? They can be totally different kinds of surveillance so you need to have different strategies to avoid them.
Also need to consider that making an extremely elaborate or onerous plan that you will never bother to actually enact, or requires a high degree of technical skill or perfect execution might not be practical. They might interfere with your purpose too much.
This page covers the basics of how to think about your own goals, fears and priorities. It's very generic but is a good overview. It's really key to think about this ongoing to avoid falling into the trap of following advise that might be perfectly reasonable for another person but counter productive for yourself.