Right off the bat, you might say that there is a voice chat. Yes, this is true, but it's not a tool that random players can use. After all, League of Legends tries to be a team oriented game, however, how are players supposed to co-operate without being able to communicate in a fast and clear fashion. There is the old option of using the chat and smart pings, but modern problems require modern solutions (*insert meme here*).
League of Legends isn't the same game it used to be, now it's fast paced, the difference between winning and losing depend on macro plays, making a decision within a split-second. Involving voice chat would be a major improvement to the game overall. As I said before, League of Legends is a team oriented game, similarly to CS:GO or Overwatch, and they use voice chat.
Regarding the problem of toxicity, that's what the trusty mute button is for.