I just contacted Player Support and I was told to post it here, so I'll just copypaste my original email.
I'm an epileptic and in the last game I faced a Garen in the enemy team.
I had the passive of his ultimate on me and when I was ulted with low health by said Garen, I was executed and thus came the execute animation.
I'm not aware if the animation of that ultimate is the same animation as for every execute ultimate (e.g. Urgot) or only for Garen, but nonetheless it was still a bit concering to watch.
That wasn't the first time I faced a Garen who ulted me when I was on low health, yet every time I experienced the animation, I felt physically sick due to the rather fast and wavelike animation that flashed across the screen.
I'm very thankful that one can turn off the flashing animation for when you lose health, so could it somehow be an option to turn off the Garen ultimate (or all execute ultimate) animations?
I know that this is a lot too ask for and I'd never expect of you to somehow implement this into the game just for one person, because so far I haven't experienced a seizure while playing League, but I'd like to play League without always worrying about getting a seizure when playing against Garen.
Thank you very much in advance for your answer!"
Are there any other Epileptics that experience some kind of motion sickness? I know the animation is VERY short but seizures can be triggered by the shortest stuff so yeah, just wanted to say that.