Shadowban of a topic: is there something like that in action here in forums?

I seem to have noticed that after a while certain topics become somewhat hidden, even if the discussion is going on: new posts are no longer notified to the participants and cease to affect the sorting by newest date, so the topic sinks among the inactive ones.
Am I wrong or is there some sort of topic shadowban that goes into effect after a while?

After a certain number of replies the topic does not get bumped to top after a new reply. I think the number is 50 but i might be wrong about the exact number.

50 is correct. see e.g. here: lichess.org/forum/lichess-feedback/forum-post-with-more-than-50-replies-stuck-behind

Good to know, tnx. Maybe it's worth a mention in some faq.