Kayle's "unique" laning was never unique at all.
You already have Nidalee, Elise, Jayce and Gnar who can change between melee/range forms. Kayle just had an ability to do that and she was mostly useless unless it was active. Her melee form did not get any benefits which meant you still had to not engage or even get close to minions if the enemy could have easily retaliated.
Nidalee and Elise don't lane, so the comparison is stupid. They're also mages, which play completely differently from Kayle's auto attack heavy style. Jayce can choose between being ranged and melee rather than being ranged for a duration. Jayce is also an ad caster (basically a mage), which plays completely differently from Kayle. Gnar is permanently ranged with his passive transforming him into a melee champion, with more CC and health. Again, Gnar plays completely differently than Kayle. Her melee 'form' isn't supposed to have benefits. It's a drawback.
That being said, Riot could buff Kayle in terms of specific target healing.
The one who she target heals gets the full benefit of her previous healing value while the secondary target gets this heal value, since the ability's flat CD and mana cost per level remained the same. This preserves her solo lane sustain while also buffing her supporting capability (i actually on occasion do play support Kayle).
Kayle hasn't been viable support in over 6 seasons. Her heal shouldn't have a second target because that basically removes the choice aspect between healing yourself or others. You'd always get both.
Old Kayle also had a degenerating playstyle.
You only needed to hit level 6 + Guinsoo's + Lethal Tempo to get yourself going on a warpath.
Old Kayle had her charms and for being unchanged for a long period of years, people have simply gotten used to her bad kit and played around it. She wasn't fun in any sense of the word, she was niche and she still had the big strength of taking only 1-3 seconds in dropping her targets if they had no hard CC to stop her.
Kayle would need to buy Berserker boots and Nashor's before getting Rageblade, which is 4200 gold on top of Rageblade's cost. She'd be well past level 6 at that point. Most Kayles did not take Lethal Tempo. This isn't even broken levels of power either, plus trying to argue if it did or not would ignore the fact laning phase does in fact exist. Kayle is best at 4 items, not 2.
You also didn't really explain how her playstyle was degenerate. You stated your opinions that she had a bad kit, wasn't fun, was niche. The only thing you said was that she can melt targets if no one CCs her, which is a statement that applies to every ranged auto attacker.