What particularly annoys me is that the sidebar used to show videos related to the video I'm watching. Nowadays perhaps a quarter of those links are relevant, the rest are useless shit.

And then youtube has the gall to suggest that if I want useful links there, I should turn on browsing history. You don't need history to suggest related videos!

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Google ads went though a similar decline ten or twenty years ago. Google Adwords used to be related to the page they were showing on, and not based on your behaviour.

You can still manually switch to a "Related" tab. On the mobile app, you scroll down a bit and the tab header appears. In the browser, there's probably a way to automate it with an user script.