I mean the ability in mobile defense.
Relief Infantry is decent-ish, but comes so late Osttruppen will be 100% useless in combat.
Oh I highly disagree with that! Deployment of two osttruppen where one has an lmg is quite a decent addition if you suffered some early wipes and they do a good enough job for capping, crewing lost team weapons and holding down defensive positions. By no means a useless ability.
With relief infantry however, you have to be extremely lucky or well timed to get even a single osttruppen squad in the ridicilously short timespan, without suffering a wipe in return. On top of that you often have to pretty much throw your squads into a grinder to suffer losses, which is a horrible approach for units with a base reinforcement cost of 30 manpower.