I was messing about in training mode, and forgive me if this is the wrong place or this has been talked about elsewhere as I've never used smashboards before, but
falling upair into an upsmash feels like a KO confirm? i'm not sure if it's random or the only way to guarantee it is to position yourself perfectly (EDIT: i just spent 20 minutes doing it and i don't think it's random at all, you have to be very precise with the hitbox)
basically you have to fall on your opponent, upair right before you hit the ground so that ONLY the FIRST hit comes out, and afterwards you can upsmash immediately and it kills at certain percents
i can't really test this out with a human player at the moment, so i'm going off the AIs DI. sometimes (a lot of times) it would DI away after the uair. but I found it was more consistent if I aimed my falling upair at my opponent as if i was trying to land INTO them. connecting the upsmash was a breeze after that.
some characters, even when the AI was set to Control (so no DI) fall out of the upsmash, like bowser.
anyway, the percents i tested were Mario at 114%, Rosalina at 100%, and Bowser at 130%
if anyone can test this with a human player before I get to, please let me know what you find
(EDIT 2) after getting hit by the upair, they can airdodge, so i feel really dumb now but i just feel like this first hit of upair has to be useful for something because it seems to do the same knockback no matter what percent the opponent is at