Her ultimate is honestly not healthy for the game.
It´s what makes Kayle...Kayle, basically.
Taric's ultimate has a 2.5 second delay and incredibly higher cool down AND has a much more limited range AND has a shorter duration at max rank.
And exists simply for the sake of existing because the new Targon lore is about as shit as Elise's new lore. And Elise's new lore is pretty shit. What the hell does invulnerability have to do with gems?
Removing Kayle's would be like removing other signature abilities, namely:
Ahri's charm, which represents her seductive nature and in a way, her curiosity. She is a relatively recent humanized fox after all.
Alistar's knockback, which goes with his humanoid bull theme.
Diana's Q, which is a moon shaped skillshot.
GP's oranges(One of Scurvy's early symptoms is weakness, or CC for an in game equivalent) or his ult, because those two are accurate with his theme and the timeline of said theme in actual history.
Those are just examples. Kayle's ult is, for intents and purposes a simple taste of what angels actually are, at least according to the bible, Riot and seemingly, Darksiders(although i might be wrong for this last one). Immortal. Plus, it´s an Ultimate ability. It´s supposed to be powerful. As outdated as it is, Kayle's lore says she's been fighting a war for 10 millennia. And probably has lived for a lot longer.
Riot can´t actually make an immortal champion, for obvious reasons, so her ult exists. If anything, the only things Riot may keep in her still quite far away VGU is her E, albeit in a somewhat different way and her ult. They are both parts of her kit and theme(Fiery, actual(not Demacian) justice delivering sword and immortality). If they scrap either, but most importantly, her ult, they better just make her a Runeterran with hextech wings instead of a winged, god-like and seemingly technologically advanced(see Aether Wing's lore) being from another planet.
A shittier kit, a shittier theme, a shittier character overall that could have had so much potential, had her ult been kept.
If Riot went for game health at the sake of thematic value, then i don´t think they would have bothered with so many long lore stories, namely the Bilgewater event and Taliyah's story.
They wouldn´t bother with Ekko's comic and cinematic or Camille's comic.
They wouldn´t bother with the launch of League's Universe site.
Heck, they wouldn´t even bother with short lores. Or any lore whatsoever.
I´d have theme over game health any day, everyday. Because, as Riot Repertoir once put it: A champion can be balanced without being healthy. And healthy without being balanced.