It could be argued that a great deal, majority even, of 1e, 2e and 4e mechanics aren't (and shouldn't be) supported in 5e. Because, mechanically, they are not the same.
Sure, 5e doesn't need to 'support THAC0,' for instance. Not relevant to whether past player options make it into the current edition.
I can't think of any that it can't do.
When the PH1 dropped, for instance, there were no psionics, there's some in the pipeline, now, still 2 years in. When the 1e PH dropped, it had psionics.
You're right that there should be no expectation 5e psionics have the utterly weird, broken, anachronist-sounding form of 1e psionics, but they're not in the PH.
I'm just stating the fact, that 5e offers more character options in its first few years than previous editions managed. That's all.
I don't understand your commitment to this this statement. It's flatly false, and objectively falsifyable. It'd be a pain to actually dig up /all/ the material that was spewed out in the first 2+ years of each of the more prolific editions, and count up the tremendous number of classes, sub-classes, PrCs, builds, kits, backgrounds, themes & whatnot for comparison to the number of sub-classes in PH+SCAG, but if you want to force the issue...
...maybe someone else will take you up on it.
I'll just stand by the fact that where 5e sits is PH + 1 supplement with some player options, and that it compares favorably to any other prior edition at the same PH+1 point in its publication history. And, of course, that it still has years to go, at that rate, to reach its 'big tent' goals, but there's no reason to think it can't get there.