The second Eternal September, directly related to the rise of social media, which I think was ultimately fueled by the mass adoption of smartphones.

Before, the Internet was mostly inhabited by people who wanted to do something specific, and were at least minimally invested - they had to actively use a computer to access things on-line. Nowadays, the Internet is inhabited mostly by people ready to judge you while sitting on a bus, because their phone buzzed them a notification that their friend retweeted a story about some stranger being outraged that some other stranger retweeted a link to your SE question.

Things got corporate.

What was once a bunch of individual sites, forums, chatrooms, etc., is now concentrated on a few companies (facebook, google, reddit,...), which are not that interested in content, but more into advertising and pleasing advertisers, and earning money, and advertisers, and ads and money, and less on their users.

This meant that everything had to be policitally correct (unless it was deemed acceptable to target a specific group), moderators got more powers, more thought bubbles and echo chambers... and now everything is going slowly downhill.

I think 2016 was a big turning point for US (and to some extent global) culture, where politics became a 24/7 spectacle for many that had been previously disengaged. I think all that energy then carried over to lots of pursuits that people didn’t take too seriously before.

The change started well before 2014 but social media mostly.

The kids grew up. It's not really the internet that changed much, but the people being active on the Internet and responsible for content and sites simply became older, more opinionated, more educated, sometimes more damaged. Society simply matured and people stopped playing around.

Humor was made illegal around 2014.

I would say it changed after 2016, the Brexit vote and Trump winning the presidency seemed to signal to the big social media companies that they needed to make changes, and this spread to many other sites too, perhaps with social momentum or perhaps simply because Google or Facebook et al can limit a site's audience if it won't act in a certain way.