This is particularly hard if you're not interested in gaming (beyond the N64 level).
I see N64 discord servers. I don't know if they're busy, but I see 'em.
In my experience on Discord you only find absolutely toxic communities, which is not very surprising, considering it's the casual equivalent of IRC.
The claim was the complete opposite that almost any objectionable behavior is labelled as "toxic".
It is hence about as undescriptive as "liberal" is in American politics. That term many years ago had a well-defined narrow meaning but is now far removed from that (or like "literally" often meaning "figuratively" in common use ).
I've got friends who say things I disagree with, they're not assholes. They're nice people with different views. Some people say things I agree with, but at the end of the day, they're still assholes, even though we share some opinions.
Any group large enough is going to have some assholes.
If you get called toxic a lot, it's not just because people disagree with what you say, it's probably how you're saying it.
Further, maybe you're not an asshole in every group, maybe your (and I'm using the word "your" here in a general term, I'm not talking about anyone specifically) personality just doesn't jive with the rest of the group, making you the asshole when you just have a personality which doesn't fit with the group which makes you toxic.
Take for example a subreddit like /r/aww which is just idk, cute photos. Someone who comes to that community and points out flaws, or seems to need to point out problems with the posts, content of posts, etc, well maybe they're toxic to that community, since it's just one of feelgood-ness. It's not that they're a bad person, or have bad opinions, they're just not behaving in a way which seems appropriate for the group.
Much like you'd hear about a toxic environment at home or in a poisonous mine, it's a very dangerous and bad place to be.
In some games, where the only way to have fun is if everyone knows all the rules and has skill and is playing seriously, one person who falls out of that spectrum can ruin it for the other(s) and thus people start to get angry very quickly and don't hold it inside. Games like League of Legends is 5v5 and highly skilled. One poor player can result in a 20-30 minute game with a guaranteed loss and no fun along the way, so it becomes very easy for people to spit vitriol at them. You get punished for leaving as well, so you are locked in for a certain amount of time, held prisoner if you will, by someone who doesn't meet your standards and whose weaknesses you can't overcome yourself no matter how good you are. Yes, this sounds like a horrible system and an easy recipe for disaster. On the flip side, if both teams are good, it can be very rewarding.
One thing I like about Heroes of the Storm, which is very similar to League, is that they solved a number of issues that results in less toxicity. It's still there, but it's taken much longer to grow and is still at such a lower level then what I remember League being years ago. I even got an email from Blizzard saying they banned someone I reported, so that makes me feel like at least something happens when I report bad behaviour. Since the game was designed to allow for comebacks and a single player's contributions can't hurt you as hard as in League, a strong team can still have a chance with one weak player. You still have assholes though and you still have rough games.
Left is a group of people that react very angry very quickly, even out of the game.