There's also a large number of skill shots that only have audio ques. Noc's Q comes to mind, where if you hit an enemy in the bush, it makes a noise, but there is otherwise no notification unless they move and spread the trail. Unfortunately, this is not a simple undertaking.
There's a ton of spells that have audio feedback but no visual feedback if the enemy is in fog of war. You'd need to go through and Identify each of them to start, and with 123 champions in the game, that's a lot of stuff.
I don't know what their back end is like, but it sounds like at this point, they have a fuckton of modular building blocks, so adding the assets should be easy.
The trouble is making the assets. For every spell that needs to have visual feedback, there's design overhead to figure out what kind of feedback they want, specifically. It'll take potentially several iterations of design and art coming up with what they want it to do and look like, as well as polishing the quality once they get it how they want it. There's likely to be bugs generated as well, and QA time needs to be allocated for each feature added.
This is just for small things like a skillshot into a bush. Figuring out what to do with rek sai and sion is another matter. It may be wise to implement a 'deaf mode' similar to color blind mode, but that would involve additional code work, and a plethora of bugs.