I didn't even know this was a thing!

There are plenty of chromium browsers like brave that will keep the feature alive. I suspect it'll just become a compile time option. I'd be surprised if there weren't enterprise customers on Chrome that will need v2 manifests for years.

The real question is what webstore will host the extensions...

Where do musify and black hole get their content from?

Another Logseq user here as well. I use syncthing to copy my note folder across machines. Pretty happy with the setup.