The current WR in knife-only any% (https://www.speedrun.com/re8/run/y8do2g5z) shoots the slime walls during the Moreau section.

The rules don't explicitly say anything about shooting them, but idk man. The goal of knife-only is to use the knife as much as possible, and the rules do say that you should only use other weapons when it's required for progression, or you're just screwing around (e.g. firing a bullet into the air, since it's not hitting an enemy).

So shouldn't the rule be that you have to slash the slime walls? Thoughts?

The rules say you have to follow the knifes out challenge rules. You can shoot anything that isn't direct damage to an enemy. You can even shoot red barrels I believe since they don't directly damage an enemy with the bullet.

Barrels are not allowed afaik. Shoot is always allowed aslong as you dont hit an enemy. Only thing where shoot to dmg is allowed, is for mifandas 60% spiritbomb. And only if u hit the ball. If u even strife her wings its reject, so be carefull. For the slimes on HC/VoS u use the karambit, what is a onehit for the walls and my prefered way to open them. On cas u might shoot with the sniper cause its faster than 4,5 slashes.