

This is network-specific. It’s been going on for a few months at least. yt-dlp itself still works without sign-in, if your network is not “suspicious”.
Always worth making sure you’re updated to the latest version of yt-dlp, but this is probably a network thing.
I don’t think you can expect any VPN to work without sign-in for very long. Google’s playing whack-a-mole with VPNs.
I’ve never actually tried signing in with yt-dlp. How easy is it to make a throwaway google account nowadays? Do they require phone verification or something similarly onerous?