No, it’s just the poor design of the new forum. Previously, the forum landing page was the typical discourse one, where people could see the recent posts and the categories at the same time:
It’s this page, with the categories on the left and recent posts/topics on the right, which works as an extreme multiplier of community engagement. The combination of seeing the recent posts, topics and avatars of the forum users familiar to you is what creates most of the user engagement on discourse based forum.
With the recent Epic community hub, which merged in a crippled version of the discourse, this has been destroyed, so it’s quite natural the community engagement will drop to a fraction of what it was.
There will be much, much less answered new threads, because people will miss majority of them.