I’m not sure what the cousins you’re referring to are, but the default categories watching
setting doesn’t accomplish the OP’s goal:
I don’t just mean there’s no UI like that. AFAIK, the only way to get a specific group (not all users) to “watch” (or “watch first post” or whatever) a specific category – and to have that happen after the user has been created – is by using a plugin, such as the one I use.
I’d be happy to ditch this workaround, but I don’t see this functionality in core. Or have I missed something in a recent update?