A simpler way of describing what’s being asked for here is that this would essentially mean supporting intersection pages for categories, same as we already do for tags:
So just like we’re currently able to do this with tags:
https://meta.discourse.org/tags/intersection/docker/email
This feature request asks for the ability to do something like this with categories:
https://meta.discourse.org/categories/intersection/support/howto
although ideally the category intersection wouldn’t need to be defined by a series of categories listed in the URL, but rather a single identifier:
https://meta.discourse.org/categories/{custom-name}
There’s not a strong use case for it here on Meta, but discourse.codecombat.com could really use this for their “Other Languages” category. That’d allow them to use sub-categories for each of their language categories, while still being able to bundle those language-specific categories together.
The Rust community could possibly merge their users and internals forums, which would be a big win.
Doing this properly does involve more UX work than the tags equivalent though.