It already is untechable, if they're grounded (Which is what Thinkaman was talking about). Falco's dair shares this property, I'm not sure about Ryu's dair but I think it does as well.
Quick note, Ganon's D-air actually is untechable due to the amount of hitlag it causes when it hits.
Really? I'm pretty sure you can tech all spikes or Meteor Smashes as they're officially called. Kind of long-winded in my opinion when you can just call them spikes. It's only in Melee where there was a spike, Falco's Dair, you couldn't tech and that was revealed to be a programming error. The rest had all spikes be techable and in Brawl, there were some spikes you couldn't meteor cancel.
As for Falco, like Lavani mentioned already, it's because of a special interaction that I think only Falco has involving his, initially, Falco Phantasm, and now, Dair. Other spikes in Smash just strictly spike unless they have a late hit like Luigi's or have different hitboxes that are called at different frames like Mario and Marth's or on different parts on whatever is hitting like Captain Falcon, Ganondorf, Ike, and Roy's. Falco's special where he has hitboxes dependent on whether or not the opponent is grounded. If they're airborne, his Dair and Falco Phantasm are treated as spikes, but if they're grounded, they're treated as non-spikes or even "anti-spikes" since both launch people up instead of down.
I don't know how this is programmed, but assuming his Falco Phantasm was like this in Melee, then I don't know why all spikes don't have this property. Yes, teching can be considered an advanced skill, especially to newbies, but that being said, it's not like a spike that can't be teched on the ground becomes broken. Smash 4 Falco's Dair spike/clean hit kills around 200% earlier now at 188% to his late hit's 256%. Sure, it doubled, but it's still pretty weak for a 13%, frame 16 hit - Utilt kills earlier by 17%. The ability to just launch people up lets Falco use it to setup in a weird, but guaranteed way instead of inconsistent since the hit lag along with it being not as strong as Ganondorf's meant you could tech it more easily.
Outside of Zelda and whoever else Thinkaman mentioned, I can't really think of any other characters who'd greatly benefit from this. Luigi and Ryu come to mind, but that's because they have frame 10 and frame 8 Dairs, respectively, but you just gave them a vertical launcher now which they could instead use something safer like Nair or Fair.