I have been told that Khri Steady doesn't buff slings because buffing all weapons is supposed to be a skillset placement perk for Barbarians. It seems like that idea died after making the new Khri, because both Rangers and Warrior Mages can buff all weapons with their updated 3.0 spells (see: See the Wind, Oath, Claws of the Cougar, Tailwind, Mantle of Flame). Paladins can also buff all weapon skills through sorcery by picking up Tailwind - there's even a tattoo for it. The entire concept of a guild not having a weapon buff to define something about it rests on Thieves not buffing slings, while all 4 of the other weapon proficient guilds buff all skills - it just doesn't work as a concept.

Throw us a bone here, let us buff slings, you're not breaking any design that wasn't already broken.

Maybe I am missing some - I was counting guilds that have native buffs for large groups of weapon skills: steady/strike, dragon/eagle, tailwind/mof, stw/cotc/oath, and rw. Of those 5 guilds, all but Thieves can buff all weapons if you assume the Paladin picks up Tailwind. Though of course as you point out, literally any MU guild can do the same via sorcery.

The only place where an exception was carved out was slings from Khri Steady. Even if you go along with the idea of limiting weapon skill buffs in this way, slings doesn't even make sense. That weapon is more on-brand for Thieves than 2HB, or polearms, or any number of other weapon skills that they can buff. It's a relic leftover from a design concept that was abandoned years ago.