Fediverse

There are around 20 projects doing groups/forums and they're coming to Mastodon (eventually - not sure what's holding them back). The only thing that makes a traditional forum different from a group is that the first post usually has a title and Mastodon doesn't support titles in posts and many social network users have outgrown them. So, like lemmy, you would need to make the starting (topic) post on an instance that not only supports but mandates titles.

Otherwise if you don't care about titles, use groups. You can post to them from any platform from within your personal timeline. We put all of our project support and development discussions on fediverse groups long ago. But you can use them for anything you want.