Personally, I don't see how this is
too much of an issue. Your voice data was already passing through Riot's servers, and anyone in the game can record your voice already.
Of course, as
Speerk25 pointed out on the webpage, now Riot is definitely storing your data, where it can pretty easily be leaked (even Google is pretty unreliable when it comes to keeping data secure, so data leaks are a bit of a problem).
Currently, I'm trying to build a speech-to-text-to-speech program to make my voice sound as monotone as Alexa, but I'm discovering that you need a fairly decent microphone for this to work (an asset I don't have at the moment).
I mostly have some privacy concerns about this. People tend have FAR looser lips than fingers.
That, and it is fairly easy to trace people from voices, if you know what you are looking for. I'm pretty sure voices are considered "personally identifiable information."
Actually, a lot of people don't see the point of doing this if people can just mute the toxic players. However, I suspect that Riot wants to do this primarily so that they can analyze the records and see if reports are valid before banning them and preventing others from hearing it in the first place.
Of course, an alternative approach would be to monitor how often a player gets muted, and then start automatically muting them if the number of manuel mutes become exorbitant. Granted, this "self-governance" feature is theoretically open to abuse, but keeping the numbers hidden from everyone should make it a little more difficult for this to happen.