So, as someone with a lot of very strong opinions about both League of Legends and online communication, I am wildly in favor of this decision. The problem with all-chat in League of Legends, particularly in the ranked queue, is that it can only hurt your team's chances to win. At best, you're wasting time typing; at worst, you're actively frustrating and distracting yourself from what is already a very difficult gam…
The question is why they decide to take away an OPT-IN feature (you can also mute single players) which a huge size of their player base uses to build their own communities and friendships.
For me this is a total surrender from a dev studio that has a history of going about problems in radical/disputed ways without regards to their own communities criticism. Dynamic Que, Twisted Tree Lines, No Voice Communication, Chat Restrictions and so much more. Riots approach to solving problems is basically "If the house is on fire better call the ambulance." They will achieve their goal to save some of toxicity but they let the house burn to the ground.
There are also way more intelligent ways to combat toxicity as blizzard and other publishers show in their games. If you flame your opponents and the bad filter catches it a message like "gg ez" would be translated into something like "I’m wrestling with some insecurity issues in my life but thank you all for playing with me." or "It’s past my bedtime. Please don’t tell my mommy.". Electronic Arts is simply shadow banning players.
The most important part is the loss of player engagement. As a player you now can't distinguish if you are playing against real players or bots and I truly believe this is the real ground for concern if you watch their dev vlog about their progress with AI bots I am 100% sure that we will see AI bots in the near future on servers with a dwindling active playerbase.