I know there's genetic backflow from neanderthals to many african populations. I found a snippet from a paid article that seemed to be saying that (some?) neanderthal-related ancestry found in sub-saharan populations is early sapiens genes that went into neanderthals, but I couldn't find a solid yes/no on neanderthal alleles in sub-saharan pops. I'd guess there'd be at least a little gene flow from nearby populations that do have small neanderthal ancestry, but that's just my uneducated guess.
Denisovans too for some of us.
And I think at least 1 ghost population of different humans that we've not found remains of!