By the “sewage” analogy you are expressing the assumption that the vast majority of what people write is outright toxic and that being exposed to it is actively hurtful.
My experience on the internet does not reflect this, this is a very pessimistic view of people, bordering on perl-clutching.
Most raw user generated feeds are not great sure, but it’s mostly mediocre jokes and mildly provocative takes from bored trolls, and that’s usually a loud minority. Most people either lurk or make a modest effort now and then, particularly in niche communities like this where most people aware of it will already be fairly deeply immersed in tech. People have better things to do than to constantly be aggressively offensive, I imagine it gets old fast, and you really need to go out of your way to write something that legitimately hurts an adult.
Sure of course there are corners that are cesspits of hate, but they tend to band together and it is quite hard to bump into them accidentally. And when you do, you just feel slightly disgusted for a second, turn back and forget about it.
Some moderation is critical, but it usually needs to only be enforced for a few bad apples, most people act with decency and common sense, even when anonymous. And yes including people with lesser means and/or from shitty countries. People from different cultures are mostly the same when you peal away superficial customs, and I find much more in common with someone of my age with similar interests from the other side of the world, than with a grumpy old neighbor frankly. At least that’s my experience.
My experience is that most forum style social media has been devoured into the reddit world, and furthermore that any attempt at making an offsite version of reddit or similar forum-like functionality is either locked down with rules that would make the Stasi blush or quickly converges on a new Stormfront forum.
The problem with reddit's panopticon moderation, with its ill defined, nebulously (and now AI) enforcement of sitewide policies, ends up repressing a negative behavior rather than refuting it, and, when people move to a similar off-reddit site, they are itching to start taking part in discourse they weren't allowed to before.
The end result is that people who are used to policing their own speech to avoid the panopticon rather than because it's the right thing to do eventually lose that moral code that was previously shaped by discourse and pushback from their peers rather than anonymous opaque moderation.
Repressing rather than refuting pretty closely models real life though.
Usually if you violate social norms people just push you out of the group and not bother explaining it to you. Not always, but usually. Yes if it is so bad it gets violent or something you will find out for sure why, but if you just show up to a friend function and start spouting off about gassing the jews or something most likely people just won't invite you back and never explain why.
Actually finding out why you were violating social norms I've found is mainly found either on the internet or from your parents when young. Hardly anyone in real life is going to bother telling you why, especially when some people are liable to act violently and there is no upside to them for bothering to explain it to you.
Socialization usually involves corrective action and nuance. A platform that will give you an AI issued permaban for saying "Say that again and I'll knock ur block off pal" about some silly topic makes people too aware of the repression and then it's sublimated in communities that approve of actually heinous stuff.