If a Rioter ever does do work on Ninja Tabi, they could have them reduce damage while airborne.
Because Ninjas can dodge and do flips in air, and have double-jumps and everything. Alternatively, you could give them slow immunity for a split second after landing because Ninjas always stick the landing.
Relevant comments (imo) and links to their threads:
Back when I was doing some work on tank items going into last year's mid season, I was pretty convinced that an effect as strong as Ninja Tabi's really shouldn't exist until like a high tier completed item. What kept me from changing it was honestly 1) there's a decent argument that it being in the boots slot means it comes at a significant opportunity cost (this assumes the other boots have equally powerful/desirable effects) and 2) it was difficult to know what effect would be appropriate here.
I'm still pretty convinced we could do better here, but unfortunately, I'm not sure what the solution is.
Link. Thread is about how Ninja Tabi might be too powerful. Keep in mind that before they ever get nerfed, Marksmen will have to be nerfed, too.
It can’t work on displacements because that would break the game. Those displacements keep you from getting stuck inside Anivia or Taliyah’s walls, or from being dropped right inside Urgot or Volibear when they flip you. It goes for knockups, too. You would drop out the bottom of Zac’s Abduction, Rakan’s Grand Entrance, and Fizz’s Chum the Waters.
And even where having Tenacity reduce displacements wouldn’t break the game, it would ruin the game for the people using them. A single boot upgrade would make Thresh and Blitz players useless. Lee Sin, already feeling bad after losing Tracker’s Knife, would have to guess how much Tenacity an enemy had when doing an Insec. Vayne have to do the same when trying to condemn anyone into a wall.
Riot kept Tenacity from affecting displacements for the above reasons—and wisely so—even though they knew melee players would love to feel unstoppable.
Link. Thread is about the overabundance of displacements in League and how Tenacity doesn't affect those. Comment makes a great point on explaining the latter.
I suggest that instead, Ninja Tabi offer reduced damage while airborne or slow immunity thereafter to lessen the pain of chain knock-ups or give a brief window where you can try to avoid them, because lots of knock-ups have slows tied to them. The way in which they reduce damage taken can be anything from resistances or % damage reduction while airborne, to a shield with a cooldown that procs when you're displaced.
There are enough other items that fulfill the need of countering auto-attack based champions (Randuins, Thornmail, Frozen Heart), Ninja Tabi might be too strong and there are none that fill the void of countering displacements, so this seems like a good space they could be shifted into. It would keep its Armor of course.
If the need for Tabi's passive comes up again, it could be brought back on a more appropriate item (e.g. Randuins or a whole new one).