Check out the newest feature of the Discourse ActivityPub plugin just announced by @angus:
[…] We just merged the PR that allows a Discourse category to follow any actor in the fediverse, including other Discourse categories. Yes, that means you can now federate a category between two (or three, or more) Discourses. There’s a few more PRs in the pipeline as part of this phase, but that was a big one.
The post on Discourse Meta has a video demonstrating the feature: