When the task description is edited, add "Edited", similar to the label for commented.
This does not block using Phabricator for task tracking, since edits are already shown in the log of what happened on the task, just not at the top.
It is tracked upstream at https://secure.phabricator.com/T4944
This task was initially reported in May, in our first days of practical use of Wikimedia Phabricator. Since then, logs of description edits without requiring an "Edited" flag have worked well in my opinion. As btrahan pointed out upstream, now we all assume that tasks can and probably will be edited. This is a different paradigm than Bugzilla, actually closer to MediaWiki.
Declining.