MT or OT warrior is, in fact, a tank. I assume you understand what this role means and imo off-tank is an irrelevant term. Many people think of OT as a tank that just sits back and enjoys the fight (maybe grabbing a few adds) while the MT is struggling to stay alive. That is the main reason there are tank swaps, both tanks soaking damage etc. But if you want my sincere opinion, its just all about skill. I know warriors that were able to tank T5(since you mentioned 2.1) and therefore dealing with death sentence which used to require any kind of mitigation, with darklight gear and others that couldn't even with i120 or i130.