Text-based chat is actually the worst possible thing for both communication and player behavior.
So, I'm fairly new to LoL (Kind of. I actually tried the game out back in 2014, and I've been playing occasionally ever since, but I only started playing seriously around September), and, being pretty bad at the game, I've obviously experienced firsthand just how toxic other players can be. I think this is the only game where other players have given me sole blame for a loss. While this seems counter-intuitive, I honestly think implementing voice chat would improve player behavior. Both the inaccessibility of having to type out a response and the inherent lack of tone in raw text can increase toxic behavior while making it more difficult for players to use it for its intended purpose of teamwork and strategy.
First of all, text based chat has the annoying problem of being unusable while also actively playing the game. This makes it difficult to use for actual teamwork - how am I supposed to discuss with my team whether we should take Baron or an inhibitor with such an inefficient means of communication? It's really only possible to actively type in chat while dead. This makes players more likely to use chat for a different, less constructive purpose: blaming a teammate for their recent death. But even if text chat wasn't so clunky and inefficient, text is simply a fundamentally worse medium for communication than voice.
Much of our communication is through non-verbal means such as tone of voice, which are totally lost in text. Communication between players can quickly get heated over simple misunderstandings such as one player asking another what they're doing and the other player taking it as a rude rhetorical question. Such misunderstandings would be much less common with voice chat, as it's much easier for players to understand each other if they can hear each other's voices.
Additionally, voice chat can make people view each other as more "human." Players are much more likely to act toxic if they see their teammates as words on a screen rather than actual human beings. I've attended numerous in-person competitions for other games, often with actual money on the line, and my teammates there are always really nice, regardless of how bad I am. While having players meet in person is obviously out of the question, voice chat can also help humanize other players. Back during SG Invasion, I regularly communicated with premade teammates using Discord, and I don't think a single player I talked to on Discord said anything remotely toxic. On one occasion, one player in our premade who didn't have Discord started blaming me for the team not getting S rank, saying it was impossible for the team to reach S rank with me on it (see my summoner icon for more details XD). I distinctly remember my teammates on Discord having my back when this happened rather than joining the toxic player in blaming me. Playing without a premade, I feel like I get flamed in almost every game. In my experience, players behave significantly better using voice chat than text.
TL;DR: As we all surely know, League players can be really toxic. Text-based chat makes players more toxic, as it can only easily be used while players are dead, when they often want to blame a teammate; it lacks nonverbal aspects of communication such as tone of voice, which could help prevent misunderstandings between players; and it makes players seem more like text on a screen than actual human beings, which makes players less sympathetic for each other. It also makes constructive strategic communication and teamwork more difficult. Voice chat could solve all of these issues.