See, the thing that always throws me, is people mention games as being "Old School" this and "Old School" that, but the mechanics of D&D 3.0 were basically a streamlined version of the Middle Earth Role Playing (MERP) mechanics, which were a streamlined version of RoleMaster, which arguably had its heyday from 1980-1994, before the RoleMaster Standard System retooled a large part of the system. Mutants & Masterminds is not usually considered Old School, but combines elements of 3.0 (which, see above), and Champions, which is definitely an Old School game as the system that came out in 1981 didn't undergo meaningful change until the 4th edition came out in 1994. Yes, for players steeped in AD&D 1E and 2E it seemed radical and (perhaps) alien, but for people who played other systems in the interim it was just the good material, repackaged and glammed up.
I understand it more if we talk about adventures, as the older scenarios definitely have different design goals from the stuff that came out once Dragonlance hit it big.