Maniphest has a half-complete Subscribers implementation with some old hard-coding. It should be fully modernized.
Pretty much all of the time, I care more about a comment being left than I do about an subscribers being updated. However, because adding a comment also adds a subscriber, I often get "@alincoln updated subscribers of X" notifications rather than "@alincoln commented on X". It seems inconsistent however and sometimes works then other way around.