thanks for that answer. I think I would disagree with you some. And really I have been putting too much thought into this lately when I should be thinking of other things like just making fun encounters and scenarios. But it is interesting if taken as a thought experiment.Mutants and Masterminds. So, yes. Mind you, that is a havily modified engine. But also mind that most every homebrewer modifies the engine somewhat....
The last time I played Old School Hack... it felt pretty much like D&D to me. So, I'd say probably yes.
For the second, probably not. "D&D" is not being used as "Kleenex" here.
I think the ambiguity of language is a useful thing. There is "Playing D&D" - using a ruleset published by an appropriate copyright holder as D&D. Then there's "playing D&D", which is playing a game that has enough of the traits of D&D as to feel as if it is in the same sub-genre as D&D itself.
And, much of the issue is right in that word - sub-genre. Folks like sharp lines on their definitions, but genres aren't like that. If you have enough of the tropes (which include, but are not limited to, the published mechanics) then you're in the genre. Genres are defined by inclusion, rather than exclusion.
Asking questions when you don't have a presupposed answer is not gatekeeping.