What's the to-go platform to host a forum?

reddit, it seems. Can be made private as well.

I know that's not exactly what you were looking for, but it's true that the discussion that used to be on private, unconnected forums is now on reddit.

I'd second this. If you don't want to manage it all yourself, you can farm that part out to DiscourseHosting: https://www.communiteq.com/. I use them for one of my forums and find them very responsive when it comes to settings things up. You'll probably need that if you're migrating from PHPBB.

For new forums, discourse is great as it's far easier to get started populating a new forum. It's less obvious that it's a sparsely populated forum as the main page isn't broken down by categories with low numbers of posts.