I went to one of the forums, went to the little bell option, and clicked mute for that forum (and confirmed that it stayed clicked after I went out and then came back in). When you do that mute option, it says “ Muted -
You will never be notified of anything about new topics in these categories, and they will not appear in latest.”
And yet, every time I go to Latest, there are some posts from that forum. Shrug emoji.