for example this semester I had a course that used a forum-like web-based tool called Piaza that was managed by the course staff

the issue is that the next cycle of students may not be able to access all the discussions that were held on that Piaza forum if the staff chooses to limit access to it.

github discussions? It is a forum-like set of topics where everyone can link the solution or post a formatted code snippet.

I was going to suggest just using git. That way everyone can have a local copy