To be clear, trust levels decay, as both Beaupeep and Leilleath mentioned. Frankly, I think you’re on a wild goose chase with the idea that you’ve had some unknown account action applied or that this is some kind of bug.
If you had a thread removed, hidden for being inappropriate, some kind of forum account strike for posting against the rules, etc., you would have not only seen the notification here on the forums, but received email to that effect.
Posting things here doesn’t violate copyright, otherwise the music video thread that has been going for 10+ years (since the old forums, actually) would have been shut down eleventy-million times already. Sure, individual artists might take umbrage at their personally created art being posted in a thread, but that would be rare, and the first step would be the artist visiting the thread and asking the artwork to be removed.
There is no way to flag or report a post for ‘copyrighted material’; we are allowed to post anything we can link within the confines of the code of conduct. I’ve never heard of art being removed from a thread due to an artist’s complaint.
Your trust level simply decayed below level 3 at some point between when you last had it and now. It is indeed down to the second, so while you might have had TL3 yesterday at noon, it doesn’t mean you’ll still have it today at 3:30. The trust level isn’t permanent.
Virtually every time people make this complaint, it’s because the “Must have read X number of threads in X amount of time” has decayed below the acceptable level for TL3.
You might try posting over in CS; perhaps an SFA can point out exactly which part of the TL3 requirements has decayed for you to lose level 3. I’ve seen that help happen before.
Otherwise, I’d just pick a thread or two from general that has a couple thousand responses in it, and browse it. Good luck.