Thanks for engaging. I don't think I need to go to a lazy consensus vote so I won't unless someone thinks necessary.
The PR is now ready for review if anyone is interested: https://github.com/apache/airflow/pull/39336 It was made more tricky by the fact that "backfill" is literally a second scheduler. But still, not as much code change as test change, with all the places we end up asserting try_number. Since we no longer will increment during task execution, many of the tests, which "run" unscheduled tasks, end up with try_number=0. In other cases, we used to verify that after you ran the task the try_number was incremented and showed the "next" try_number (in reality it just appeared incremented because it was returning _try_number + 1 since it was not running). Anyway...
