I would like to express strong interest in this feature also.
The plugins linked are special-cased - and exporting the user lists as suggested here is not a real-time solution (and not automated).
I strongly support the use case of being able to create categories that are “auto-watched” by users. For instance, I - like others - have been using Discourse for a class I’m teaching as a Canvas/Piazza replacement. I ask all students to read the “Announcements” category, but short of collecting their emails and creating a separate email list I have no way of pushing important announcements to them via email (something that Piazza did provide). I’d be ok if auto-subscribed users could unsubscribe themselves even.
The only currently available alternative is to ask everyone to watch this category.