NPM staff claimed they were fired after trying to unionize

My bad, I didn't saw it. And it seems the links were different (extra # in the other link), so this submission was accepted.

If I still used npm I would be very concerned that it will go the way of SourceForge.

You mean yarn?

yarn is just an interface to the vast npm ecosystem.

My bad I thought they use their own CDN

Yes they do. They have their own CDN on top of npm. Potentially they could fork the registry if they wanted to. (AFAIK, they currently don’t)

Extremely concerning.