Just like the title says. I think we can all agree that the skins nowadays are a lot more sound and FX heavily differ from the classic looking champions that we all know and love. Some skins can actually make the champion look slightly bigger than it's normal counterpart (or smaller in some cases), these differences between the skin and it's default counterpart do affect gameplay in the way that the player has to memorize how that skin's effects and sounds are like so you can play around that champion. In a way I think that's cool since it's like you're relearning about the champion and that can be a nice refresh since it kinda feels like we're breathing new life into it.
But all in all for people not willing to learn a skin's new gimmicks either due to a lack of time from a busy schedule or pure laziness, they shouldn't be punished for mistaking one of that champion's abilities with a different one. Like how the new God Staff Jax skin has a small grey mirage with his Q - Leap Strike. In a split second decision I imediately cleanse thinking that he used his ability E - Counter Strike mixed in with Leap Strike. Only to waste cleanse on nothing so that he may stun me afterwards.
Having an option turn these cosmetic skins ON/OFF on the menu would be perfect to avoid situations like these. By turning them off, I don't mean to impact what my opponents or teammates see either. Just what the player that changes it should see.