Being profoundly deaf myself - although I wear implants and can hear relatively well with them - I believe that this change is an important one to make, and that Riot should consider it.
Sound effects are among the strongest cues in League - they warn you about an incoming enemy ability, when a turret was switched focus to you, and even tell you whether or not you actually last hit that cannon minion (since we all know they're so important). Being deaf basically takes away half of the gameplay, since sound effects not only are indicators, but also include regular game sound, like the announcer, background music, and champion lines (yes, I listen to those). As a result, giving deaf players visual options to make up for the sound effects they cannot hear - at the very least, the sound effects that impact gameplay - is a step Riot definitely should take.
About 1 in 20 people (I'm no expert on statistics, but I believe this is about right) have some form of hearing loss, ranging from mild to profoundly deaf, and tens of millions of people - possibly hundreds of millions now - play League (in 2014, 27 million people were reporting playing per day, and that was 3 years ago). So, it stands to reason that there are at least a significant number of deaf players in League, and therefore Riot should create visual options to replace the sound effects they cannot hear. As Kerkoc said, disabling the sound when the visual indicators are activated is a fair tradeoff, so the combination of the two doesn't become an unfair advantage to hearing players.
This is a gameplay update I personally believe Riot should implement, and I hope that someday they will implement it. After all, they have color-blind options, so surely the next step is to cater to the deaf player base, and improve their League experience.
