It looks like DI down would beat it well. It also doesn't seem like it would work with heavier characters. Tell me the results of your research!!!
You're right, DI down can safe you from this but there should be a percentage where it doesn't anymore. I'm going to practice this now.
Edit: It looks really easy, but it's not. I got it to work in practice mode, this is stuff I found out so far.
-On Robin Z-air-> Footstool seems to be working on 36% (Comfortably at 38% for me). This is without DI so add a couple percentages for DI.
-General speaking if D-air doesn't pop people up it will jablock.
-D-air pops Robin up on 86%
-This combo works without DI between 38-86% on Robin.
This means that at higher percentages where you can DI all you want and still fail it's guaranteed. Which should be around 50ish%? The range on this is too high to not be a true combo imo. Learning this to extend your footstool shananigans after D-throw footstool stopped working may be the easiest way of landing this.
Edit 2:
More stuff. This doesn't solely have to do with weight! Fallspeed is a huge component as well.
What I so far found is this.
-Fallspeed determines whether this combo works or not and how easy it is. Because this works on Bowser, the heaviest character in the game. But doesnt Fox or even Cpt. Falcon.
-Following
this list it starts working on Metaknight.
-Weight determines how much percent is needed for Z-air footstool to work. On Bowser Z-air footstool works at 60%
-Same goes for D-air. D-air jab-locks longer on heavyweights for example for bowser it's 101%
- This combination makes characters like Jiggy, Kirby, Rosalina, Peach and G&W extremely vulnerable. Low fall speed and low weight makes it "easy" and doable on lower percentages.
Edit 3: Apperently King DeDeDe is an exception because Z-air footstool works. So now I'm confused....