I wouldn't say people have to mute all. I wouldn't even say a lot do. In fact, in my past games (from the past two days), I can remember a total of 1 person who didn't say anything the whole game (but they still used pings). I'm not sure if they were restricted and self-muting, or whether they had muted all - it seemed they were still coordinating with us but that could just be luck, not necessarily them reading our chat/pings.
My point is that most people don't actually mute all - at least not by default at the beginning of the game. If my team is tilting the fuck out of me I'll do it, but that's incredibly rare and only later in the game when it becomes clear to me they aren't doing anything useful in chat.
Overall, I'd say 90-95% of my games my experience is chat is completely positive, carefree, and helpful to the team. Another 5% ish of games it's mostly positive with maybe a couple tense exchanges or one bummer player. Very few, as in much less than 5%, are this "toxic cesspool".
I have friends who see it differently. They think that toxicity is all over and super prevalent and everyone is toxic. You know what I found a few years ago? I experienced more toxicity when playing with these friends. It's so long ago I don't wanna estimate percentages, but I'd say it's easily 2-3 times the amount of toxicity. It's not like I was acting any differently and we were at a similar rank/MMR... the only difference is that one friend in particular tried to be really good at "avoiding the system" to still be toxic - things that they thought wouldn't get caught by the systems, and/or things that aren't technically prohibited like spam pinging, using emotes, intentionally taking first buff, etc.
I stopped playing with that person after I realized they were the cause of the toxicity I was experiencing in my games. The moral of this is that if you find yourself getting tilted and experiencing toxicity in a lot of your games, it's more likely than not that you're doing something to cause it. You may not be doing anything wrong - but it still may be causing something to happen. One example is that comes to mind is getting rear-ended at a stoplight. You aren't doing anything wrong by braking a little hard because the light is yellow and you don't think you'll make it. But had you not braked, the guy behind you wouldn't have had the chance to run into you. This actually has happened to me in real life before - and while I'm still careful near lights so I don't have to brake hard, I'll freely admit that I've accidentally come very close to running a red light (if I haven't actually, not sure the exact rules town/state level and stuff) because in the back of my mind I'm thinking about whether the person behind me will be paying attention. Doesn't help that half the drivers in my town have their phone literally rubber banded around their head in front of their eyes (ok, not literally, but hopefully point is getting across lol).
So, the moral of this whole thing? While you may not be doing anything wrong to cause games to become negative/toxic, it's worth taking a look. Remember that everyone opens up League with one mission - to enjoy themselves. Whether that enjoyment comes from playing with friends and dicking around and not really caring, or whether it comes from "tryharding", everyone wants to have fun. There's going to be trolling people - but there's really no way for you to know they're trolling unless they say it in chat. Give people the benefit of the doubt. People make mistakes, accidents happen, buffs get last hit by a Darius support bleed after the jg got warned "please smite it watch for bleed"..... shit happens. Hell, sometimes shit literally is being dumped on you from buckets repeatedly above your head. Don't sit there and wallow in the shit - move on from it and don't let yourself get tilted.
Fuck, now that I say that, it sounds really.... cliche. "lol just dont tilt bro hahaha wow gl with that". It's not easy, I know. What helps me is just asking yourself why. Why does it help to get mad at the buff getting taken? Are they going to magically give the buff back? If they did it intentionally, they want me to get mad, so why should I give them what they want? And if they did it accidentally, I'm sure they feel just as bad about it as everyone else does, so what good does it do to just keep everyone feeling bad.
Last thing I'll leave with - I was playing Ashe support recently because a teammate intentionally banned my Senna and repeatedly called the teammates names when they (not me even, because idgaf really) asked him why. So I figured I'd play another marksman support that's fun for me and could still win us the game. Enter the player intentionally trolling our team by doing bad ganks, intentionally wasting smite on dragon before the end of it (i.e. they didn't do it for the health regen or anything), and AFK at multiple points in the game. They were trying to piss me off. I didn't care. Said nothing in chat. Actually ended up getting the ADC super fed (I think combined we were like 11/2 in lane or something with a decent number of assists too). We lost that game, which is probably what that jungler wanted. But you know what I bet you tilted that jungler even more? How the rest of us just kept playing and didn't give him the attention he wanted. He could've reported any/all of us all he wanted - but he will never get the satisfaction of knowing he "baited" us into responding.
So I guess if it helps you... just be petty in knowing that by you not being a jerkass, not only are you avoiding punishment, but you're also giving a big 'fuck you' to anyone actually trolling because they won't get the gratification of pissing you off.