Echo doesn't test the expected behavior for different kinds of reverts (manual vs undo vs rollback). This led to an accidental change in behavior when working on T397103.
Original Report
We changed $isRevert = $editResult->getRevertMethod() === EditResult::REVERT_UNDO || $editResult->getRevertMethod() === EditResult::REVERT_ROLLBACK; to $isRevert = $event->isRevert();, which is not exactly the same. As a result, manual reverts (i.e. viewing an older version of the page, then clicking edit and save; not using the undo or rollback buttons) will now be treated as reverts by this code.
This doesn't affect the revert notifications themselves, since they are sent elsewhere, but it will affect some other notification types that check whether an edit is a revert before sending notifications: at a glance, this will affect the summary mention notifications too (they are not sent for reverts), possibly fixing T373750 but making T340883 a bit worse; and page linked notifications (also not sent for reverts), apparently fixing T148949.
We also need to add test cases to this.