In case of YT it is likely a mix of multiple reasons. They stop playlists on this screen: https://www.hollyland.com/blog/tips/why-does-this-the-follow... . Apparently music is no longer advertiser friendly. Detecting ad-click fraud is easier when users are at least pseudo-anonymous. Warnings about "ban for using adblock" is also not very effective when people could watch video in new private window.

I have no idea, but I noticed that you have to login to GitHub first before you could view any page. Surely it has nothing to do with adult content, right? I think it has to do with LLMs / bots.

And if the website contains erotic content (like YouTube), they are supposed to lock you and verify your ID. This is why all erotic content is getting filtered on X.