That said, I think there are two things worth noting about that site:
1. The content was mostly slop before AI became a thing, AI has just increased the amount of it tenfold. 2. It's still not the worst content on the site. As one check of LinkedIn Lunatics shows all too well, there's a depressingly large number of people who seem to treat the site like a personal diary, and rant about their political views on what's supposed to be a business focused network.
At this point, LinkedIn is basically Twitter/X wearing a suit and tie.
As for the other sites mentioned... yeah, those track too. Medium and Reddit were already being gamed like there was no tomorrow before AI entered the picture, now it's just become even easier for the parasites to flood them with garbage. Twitter/X is basically the wild west at this point, and Substack seems to appeal pretty heavily to the types of people that absolutely love AI and everything about it.