Boost Kick is not and never has been the issue. The issue has been her UAir's combo potential. While we're at it, there's her safe aerials even if they're not Sheik safe.
One thing that's always irked me is why is her DThrow so much better than her other throws that it makes them pointless? They don't even have much in the way of positioning value like Cloud who can't really combo out of them.
To be quite specific, the issue with zss is the combo of uair and boost kick, in that she carries you basically 75% of the way to the blastzone before her launcher hits. This negates a major benefit of being a heavyweight, which is reduced knockback, leaving them as big easy targets. An interesting tweak ive heard is to make her boost kick function like pkt2, where it hits and its distance is severely reduced. Rather than a straight nerf, they could up the kb very slightly so it actually kills better off of reads, but the combo doesnt take you right to the blastzone.
Theyd have much more freedom with a rage modifier on moves, or a hitstun modifier, but i digress.
Boost Kick killing vertically wasn't just some thing they cooked up. It's a very important aspect of her character in, you guessed it, FFAs. Like, it'd literally ruin her character if they made it kill horizontally. Unless it launched backwards, which wouldn't help 1v1 balance as much as forwards...
It's hard to turn down the aspects that are overpowering in 1v1s without outright crippling the character in FFAs. Meanwhile, not sure how often screen-length needles help when there's almost always going to be somebody a third of FD away max. Edgeguarding I guess?
Hop on FFAs online and abuse boost kick. She isnt just decent, she is clearly abusive in that mode due to the efficacy of boost kick (and flip kick). With her mobility and good survival skills, when you hit rage she can basically steal kills from anyone on the map. Zair is a disruption machine, dsmash safely fishes for insanely easy early kills, paralyzer is similar... she isnt just a FFA player, she is one of the best and an absolute monster with little effort or adaptation.
Edit: good survival skills refers to the player, zss is rather delicate. But good drifting makes punishing her significantly more difficult than others missing their (weaker) kill moves
Now that i say it, this might be a good way to get boost kick looked at by ninty. If you can prove its broken in practice (for FFAs), then it deserves it anyways (for FFAs).