To be fair, you dont need tanks in leveling dungeons on retail either. I just had a full 4 man gnomeregan run because tank left instantly and LFG couldn't find a new one until last boss. We cleared the dungeon with kiting and warrior and hunter pet tanking.
Not sure where you start to need an actual tank, but would guess that at wotlk/tbc dungeons.
That said, I think it irrelevant in this discussion, because anyone claiming classic is more complex than retail in any form or shape is a derpy derp.