So if trash talk is Toxic and damages the game experience...
...why does all chat exist? There is literally no other reason to communicate with the enemy team, other than to say "GG". You are playing, you shouldn't be wasting time typing to socialize, it's against the rules to help them. Literally all the /all chat does is encourage toxic behavior, feeding, etc.
Edit: Custom games are an exception. Only other reason I can think of is money, people just love watching their favorite streamers be toxic af then whine when they get banned.
I feel it should at least be disabled in Ranked play. There is no place in ranked to be taunting enemies, shit talking about your team to the enemy Garen who doesn't care, or wasting time on arguments instead of playing.