If you look at e.g. T74928 , when gerritbot updates it to say "Change 168761 merged by jenkins-bot" it leaves tag # "patch-for-review" in place.

It's not as simple as "when jenkins-bot merges a gerrit patch for TNNN, remove the tag", since a task can be associated with multiple patches in gerrit, particularly if some are cherry-picks or backports. Computing an accurate count of outstanding patches by reading the Maniphest task seems impossible, maybe

• The bot could query gerrit to see if there are any open patches for the task (and its BZ bug) before removing the tag
• The bot could add another tag #patch-merged and humans can query on both tags to find stuff to do or tasks to clean up. But querying for the string "merged by jenkins-bot" is just as good.