The reason I mention using a subset is because there may be performance/storage concerns on the server with managing RSS feeds for each user, or even performance concerns for the users retrieving the feeds.
For example, I might set myself to watch every category, which with no limitation would lead to my user having an RSS feed containing every single post. For the server, this one RSS feed would either be cached and effectively double the text storage use or generated every time it’s requested and use a significant amount of CPU time.
Additionally, such an RSS would potentially be very large, taking some amount of time to download and then be a lot of XML for the client to process, possibly resulting in several minutes of downloading and processing before your RSS client can actually do something useful.
In a Discourse community with users very likely to embrace RSS, there might also be bandwidth concerns to consider.
In regards to your original question of whether it’s currently possible, I think the answer is no. As you’ve mentioned in relation to Reddit’s method, this will require a mechanism to both uniquely target the appropriate user while also avoiding leaking information about the topics a user is watching, which for this sort of thing is commonly done with a unique token generated for the RSS feed URL.
Because of this, if the work had already been done for such a mechanism, I would expect to see it used somewhere on my profile pages, such as with an RSS link on the Notifications page or a section in Preferences for creating RSS feeds.
In terms of getting this functionality added, the best thing to do will probably be to flesh out the details of how this would work for users, ideally with details of how it could work behind the scenes as well, and then either:
- Update your original post in this topic with those details and then hope someone wants to take up the task of adding it to core or creating a plugin; or
- Create a new topic in / update this topic and move to Marketplace if you are able and willing to fund it being added to core or a plugin being created