TL;DR: YouTube sends a Cease and Desist email to the Invidious team for API abuse (even though they don't use the API.)
I also want to point out (since nobody else has) that YT has done shenanigans like this in the past. Invidious is not going down without a fight.
UPDATE
As of 2 weeks ago, Google has begun blocking multiple instances from accessing their services. This also includes my own instance and appears to be done alongside the C&D. (Note: I have not received one yet.) I'm currently playing whack & mole with Google and will be moving between several different servers. Some others are going through IPv6 addresses (which didn't work for me.)
Aw man. This is how I watch YouTube videos :(
I love these privacy-friendly front ends for big tech websites. I guess it was only a matter of time before Go.ogle started fighting Invidious/Piped instances.
If you haven't already checkout the plugin called "Privacy Redirect" for Chrome/Firefox. There's also an android app version called UntrackMe (I think?). They basically intercept YouTube/Twitter/Reddit links and redirect them to privacy-friendly instances.
Yeah, it's called UntrackMe. I use LibRedirect, works well enough and supports a wide range of services.
Thanks for sharing. Yeah LibRedirect does support a whole lot more! Going to switch to that now.