User subscription and permissions
As Jorm said in April, that feature-concept is a request from the Ombudsmen..
Also, keep in mind that mediawiki is designed to be adaptable (i.e. with options/toggles throughout, with different defaults at any given wiki based on their own needs), and is designed so that it can be used by thousands of wikis, both WMF projects and others.
Yes, I know - which pretty much informed my first sentence. I suppose it depends on what a "subscription" would include - if it includes one's entire feed including all the discussions on all the talk pages one watches whether or not one actually participates, frankly nobody in their right mind would approve any subscription requests. It's another matter when it comes to places where a user actually posts. That discussion absolutely must be publicly available, but I'm not seeing anywhere that describes where that information is available on something comparable to a user contribution page.
The user subscription example is just that: an example. I wrote it up by request from the Ombudsmen. The version that's out there is bizarrely more transparent than the initial one (which had all sorts of weird privacy-related events).
All discussions are public. Subscribing to a user just makes it easier to track them in a single place (instead of going to watchlist/contribs/etc.). The request was an exploration into the feasibility of openness regarding "followers". It clearly doesn't work when one is tracking a vandal, though, which is why I think it's doomed (I don't know if I wrote about that or not).
Either way: an exploration, nothing more.