Most forums were awful phpbb nonsense where it was impossible to find the information you wanted. Search always required logging in and the only interface for very long threads was paging through them 10 badly laid out posts at a time.
No thank you.
Of course modern forums are better. Disqus is ok, and D's forum software is arguably the best thing to come out of the D project.
In any case Discord is not a replacement for forums; it's a replacement for IRC.
I recall running into the odd one or two like that - I have vague memories that it was because searching was _slow_ and it didn't take more than a few people using it to bring the site to its knees. Restricting it to logged in users helped, and I think I saw some sites restrict searching to only post bodies for a period too, for similar reasons.
I haven't run into it recently, but I have at least vague memories of it happening in the past.
Just have a browse. It's so fast! Also very clean, not stupidly sparse layout. Proper markdown support. I believe it also acts as a mailing list/newsgroup somehow but I'm not sure on the details of that.
Spammers and similar bad actors. :(