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It's not something "they" do, it's an option that any video uploader has. Of course, you can sideload unauthorized YouTube viewers to get around this.

Yep, they still do that. I run into a few of these videos every now and then. It tells me to "save to 'watch later' playlist to watch on your pc".

They still do for some content in Canada, at least, with messages along the lines of "UMG Music has not authorized this video for mobile distribution".

Spotify has different rules depending on what kind of mobile device you're using to listen. My phone is significantly more restricted than my tablet or PC.