I’m too old for this to matter, but is somebody really expected check the ToS of the server the community they’re posting in is on, or is this just a CYA kind of thing?
Yeah just ignore and move on. They probably just put that there so they dont get sued for making porn available to minors without warning or something.
You should understand the rules of the places you are posting to, yes.
This is why "let's pretend this is centralized social media and ignore the fact that we're all on different websites" is a bad idea, actually. You don't get to parachute into someone else's house and expect the rules of your own home apply.