The thing about 4chan is that it's got a shitton of boards, and those boards are like kind of like, say, subreddits - they're pretty self-contained and have distinct communities. So, say, you have places like /pol/, but you also have places like /tg/, the "traditional games" forum (i.e. Magic enthusiasts and RPG players) which turned into a bubbling hub of creativity and collaborative storytelling. (and shitposting.) 4chan was
abrasive everywhere, but it's hardly
vile everywhere.
The experience I found on 4chan is pretty unique, and I don't know how well I can describe it, but I'm really glad that there was somewhere like it.
That said, 4chan's culture definitely started to decline around 2014~2015ish, around the time of Gamergate and the Trump campaign. Whenever 4chan's "hub of vileness" reputation starts getting really public in the news, the result is that a bunch of new people who think "hey, that sounds like exactly my jam!" start flooding in. And those two events both drew a huge amount of attention to 4chan, and not the kind of attention that brings good people. (It didn't help that a huge chunk of the old userbase fled around the time of Gamergate due to controversies about it on the site itself.) /pol/ was always a little leaky, but it started to get a lot worse around that time. Not to mention that /tg/ itself was already on the decline because of the positive reputation it had gained elsewhere on the Internet as various stories originally found on it were posted elsewhere, and so it increasingly became full of people who wanted to hear those kinds of stories but weren't actually RPG players themselves. There's a lot less positive to recommend the site these days, and a lot more negative.