Are you using a threaded e-mail reader? If so, do you care if discussions are changed to be flat (like Discourse)?

I use Gmail which I think supports threads, but I subscribed to digests so no threads for me.

as no one in their twenties or earlier wants to use a mailing list

I'm not from that demographic but I also don't want to use mailing lists. But I'm not a fan of 'forums' either.

What is important to you (besides that you can continue to use your email client)?

1. Main reason why I can't use the mail list: I don't want to subscribe to everything happening there. Probably there are email clients that make that easier, but that already excludes people who don't use such a client. 2. I subscribed to a digest and there is still no way to post to a thread without being subscribed to the mail list before it had been started. And even for one who had been subscribed it gives a wrong address to post to. 3. There is no way to link a post or thread. Inside the mail list itself or anywhere else. What are the options for a person who is not on the mail list? https://lists.boost.org? https://lists.boost.org is a user experience nightmare: 1. You can't post, only may create a new thread by mailing to the mail list. 2. You have to click on every message to read it. 3. In a threaded view you also can see only one message at a time, and it is easy to miss parts of the thread by clicking the wrong button. 4. Google and other search engines for some reason have indexing issues with it. Even if I know something was on the mail list or there is a comment in Boost's code with a link to a dead Gmane I had great luck finding absolutely nothing with multiple web search engines. 5. There is no code snippet syntax highlighting. 6. It doesn't interact with itself, or anything else either.

implement our own web-based forum

I don't think it's a good idea.

The other alternative is to do a clean break and deploy something completely new, which would be Discourse. The downside of Discourse is that it does not support threaded discussions, and because of their philosophical objection to hierarchical conversations

From my experience with Discourse a post will contain a list of replies to it which you can jump back and forth. I don't see a great value in highly branched discussions, it is impossible to follow even on things like Hacker News or Reddit, just create a new thread. And...

There is GitHub Discussion. What is wrong to enable Discussions on https://github.com/boostorg/boost and use it? AFAIK it supports posting from a mail client and subscribing to it to receive everything or just subscribed threads by emails?

* Written in Ruby, a dying language

GitHub is written in Ruby :-)