let me clarify
float cancel and auto-float are two different things
but both are related to peach's ability to float.
The auto-float to my knowledge is a glitch. the float cancel cancels the frame lag in her air attacks, making it easier to combo.
they did not purposely give peach the ability (again to my knowledge) to instantly float on the ground in melee (auto-float). But they purposely added it to brawl.
i guess both can be considered a glitch
You would think float canceling would be a glitch as well, since the amount of frames Peach's aerials lose are quite a bit more than L-canceling. I just don't feel it's the kind of intentional AT even Melee would of had.
'Course, I don't have any information to directly confirm that... I'm only speculating it's origins.