Recent backroutes seem to be confined to four levels - Manic 4, Delirious 10, Delirious 16 and Delirious 19. Of these, Manic 4 was a new addition in a recent test version update, so it hasn't had much time to undergo de-backrouting compared to the other levels; and Delirious 10 hasn't had new backroutes in a while now. Delirious 16 has also had no new backroutes since the latest update, but it's a bit soon to rely on that just yet.
(But as an example of this: Fixed a new backroute on Delirious 19, accidentally brought back an old one in the process.)