Riot Games released a statement briefly describing the end of the "all" chat. This means that /all chat will no longer be activated in any game mode in the future.
The change comes into effect from patch 11.21, this patch will be released on the live server on October 27th. With this, Riot Games wants to put a stop to the toxicity between teams, but at the same time they know that this won't be enough.
While /all chat can be the source of fun social interaction between teams, as well as some good-hearted banter, right now negative interactions outweigh the positives. - Riot Games
But they also emphasize that this will be the only function that is being removed. You can still chat with your opponent in the post game lobby, also emotes and taunts are of course still allowed in the game. The team chat also remains activated, because according to Riot Games this is simply too important for communication.
With this, Riot Games is taking a drastic step to improve the player experience. The "flame" is one of the main problems in LoL. For years players have complained about toxicity in the game.
Many players have long since deactivated all chat via the options because it is sometimes unbearable. For the remaining players that play with activated all chat, all chat will soon be switched off forever.
You can read the official news from Riot Games about the removal of all chat here: