As Bitcoin-Core already uses GitHub, another possibility is to use the new GitHub discussions feature. We increasingly have been using this at Blockchain Commons as everyone is using already using GitHub. We have also created some GitHub actions to backup discussions so that GitHub will not be a central point of failure -should be possible to create a static page archive using GitHub pages (but have not had budget for that).

For instance, here is the GitHub discussion area for wallet developers working together on Bitcoin wallet interoperability specifications: