If mysql handled columns declared as "ascii" as utf8 that would be "compatible" in the sense you're describing. I think it would be fair to say "mysql ascii isn't actually ascii" in that case though.
I see your point, but in that case I certainly wouldn't say it mishandles ascii. Defining some undefined behavior is very different from changing existing behavior.
Plus that's a different scale of change because it's going from fixed width 7-bit to a variable width scheme.
