The real question here is what would yall do AFTER untechable stomp?
I know twice I've hit porkchops with a stomp at the center of FD when he's at upper 20s, and he didn't DI, so I did untechable stomp (he DI's the second stomp, probably because he was trying to techroll bc who expects to get popped up again?), and then fair, and hit his phantasm with uair ftw. stomp, stomp, fair is 59% fresh LOL so it essentially puts them at 90% after the fair if you stomp them around 30%. Truth is, I could have untechable stomped AGAIN because he DI'd. definitely if the first one is at mid20s. You just have to get the dair late and dash fast af to follow their DI.
This is something kinda like the cg, where unless you have the timing down, you'd definitely want to just grab instead. But I actually think this is an optimal punish in many situations regarding fastfallers. Falcon gets straight booty blasted by untechable stomp.
I've hit marth and peach at 0% with it (1% won't work I'm certain) but not sure if guaranteed. When they miss the DI on a fresh dair. I'm certain a pal dair or a slightly staled dair would make it guaranteed.
Sheik it's guaranteed like 0-2% but obviously just GRAB lmao
Side-b to untechable stomp works on marth and Ganon at 0%, DI dependent.
There are other setups I can't think of right now, and others I haven't discovered. I have landed it randomly quite a bit, since after practicing it so much I can quickly realize when it's possible and go for it, as I know what to look for.
You can practice with visual cues just by stomping an uncontrolled falcon repeatedly and trying to pop him back up each time, timing the stomp hitbox for as soon as he lands, during the 2 frame window (guess) that you have before the game reads him as airborne again (when he bounces).
Edit: I've also hit luigi at super low%, first stomp on stage, untechable stomp on platform.