A few I used to frequent a lot, mostly around 10 years ago:
Brooklyn Vegan comments section - lots of arguing and silly jokes about hummus. Was pretty entertaining in an immature way.
Letsrun - I was obsessed with my own amateur attempts at distance running, as well as professional running, and it could have lots of good discussion as well as funny threads. But it slowly became a haven for alt-right college kids, due to its very lax moderation. It all came to a head when Trump was elected and I never went back. I dread to think what it’s like wrt Covid discussion.
Lonely Planet Thorn Tree - I was an early adopter on this one, posting from about 1997. I got a lot of great advice, particularly for a long South America trip in 1999, when information was a lot more scarce. Now it’s so much easier to find out travel info online that the forum is pretty redundant. I assume Covid has completely killed it off (along with most of the travel industry).
Now the only forums I read besides DiS are the Cambridge and Learn Spanish subreddits.