Well, that's just part of their YouTube Red service they're trying to sell.

I use NewPipe[1] for that. It has background and popout modes, including a queue for the background mode, and an option to speed up the video which is perfect for lining up a couple of lectures and listening to them as if they were podcasts.

[1] https://f-droid.org/en/packages/org.schabi.newpipe/

YouTube used to be able to play with the screen off, but Google has locked this behind a YouTube Red subscription at $10/month.

Can't watch ads if you screen is off!

Thanks for the info. Although the reason is even more stupid then I thought...

It's stupider than you think. If you're a Red subscriber and you visit, say, the UK, then suddenly you can't play videos in the background any more.

It's even stupider than that.

When I first subscribed to Google Play Music, in the UK, it included "YouTube Music Key", which had the same feature to allow YouTube videos to play in the background, and it worked just fine in the UK.

Then they rebranded YouTube Music Key to YouTube Red, and since YouTube Red isn't available in the UK for legal reasons unrelated to backgrounding YouTube videos, they disabled the backgrounding feature in the UK, even though UK subscribers are still paying for a YouTube Red subscription (that cannot be used in the UK).

OK, stop. You've convinced me. Software programmed by attention merchants (advertisers) and IP prostitutes (lawyers) instead of engineers is stupid.