All 'adversarial compatibility' from projects like Nitter, Teddit, Invidious, and youtube-dl go out the window. Any archive site (archive.org, archive.ph, etc.) can be blocked by sites requiring attestation.
And just like the book industry was terrified of piracy and were 'rescued' by Kindle, so too will journalism outlets that can't find a business model flock to Google to save them.
This is going to be rough.
What will happen if such a thing actually happens is that the underground market for "trusted device" farms grows, not too different from what's currently already happening but possibly at a far larger scale. Of course, that means the financially motivated scraping services still keep going while the honest individuals wanting user-agent freedom get screwed, just like with many other forms of DRM...