Excluding the Threadiverse?

Hmm.

Probably Reddit, if one goes by the amount of content. I mean, I still search for answers there. I just won’t log in or comment any more.

If it’s “what forum system would you use if you did want to comment and the Threadiverse is excluded”…probably Usenet. Has an ecosystem of good third party client software. I dunno how much discussion is still happening on there these days. Spam kinda did a number on it back when, but incentive to spam rises with userbase, so I assume that places like Facebook are more-appealing targets. Oh, and binary newsgroups hammering servers were also a problem.

Most Usenet access these days is commercial and not typically bundled with ISP service any more, though last I looked, eternal-september.org offered free accounts for non-binary newsgroups.

checks

Sounds like they still do:

https://www.eternal-september.org/

news.eternal-september.org is a private project providing free access to text-only Usenet News. The server has a 1GBit connection to several Internet backbones and is integrated into the Usenet via more than 40 peers.

EDIT: List of providers:

https://www.usenet.org/providers

In the past, Supernews and Newshosting were major service providers in the US. I’m pretty out-of-date on the state of affairs, though.

EDIT2: Looking at that list to cue memory, Easynews and Giganews have also been major providers in the US.