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.