I know this was something you could do in 9b, and apparently it's in Beta too. I did this once during a super laggy match, and while it wasn't that useful since I did it on the ground, it could be really useful to learn how to do elsewhere.
I can't find any videos on the technique either. Does anyone want to detail this if they know how to do it?

When Masky first discovered and described the behaviour in 9b, I took the opportunity to examine the technique in full detail.

I don't remember the EXACT details, but within 7(?) frames of coming out of hitstun you must input upB. Then, within that 7(?) frame timer that I described, you can press down and your upward momentum will be reversed. If someone was technical enough, they could alternate between pressing down and not pressing down during those 7(?) frames to get movement that alternates between going up and down. It was also only possible with a certain subset of characters, and Mario was the only one that had a cool bounce when he hit the ground during his falling upB.

tl;dr upB out of hitstun then quickly press (and hold) down