Either blocked by T120013 (if upstream pull includes changes from 2016-01-20) or its followup instance.
Since https://secure.phabricator.com/T6183 on 2016-01-20 "you no longer need to be a member of a project to watch it". "Subscribe" has been removed entirely, only "Watch" left, according to https://secure.phabricator.com/T6113#154746 .
"You now opt out of project member mail with an explicit Disable Mail action on the Members page." according to https://secure.phabricator.com/T10180 ,
plus project members don't get spammed anymore by project member list/icon change notifications.
To also update https://www.mediawiki.org/wiki/Phabricator/Project_management about subprojects if available - what do they actually provide when it comes to functionality?