prettier --list-different "**/*.{ts,js,mjs,json,md}" does not output anything. I don't know what to modify.

Try running yarn lint locally.

But also, doesn't this solution try/fail for every dependency? i.e. if you need to look up 20 dependencies, it will result in 20 * 404 errors in addition to the 200 * 200 OK?

Indeed. But a Gemfile can contain gems from multiple sources. We canno't know in advance if the gem can be fetched from api/v1/versions or not.

I will do more tests locally as the unit tests broke.

I'd wondered if we could detect when a particular registryUrl does not support the traditional endpoint and then "remember" that for subsequent requests

Maybe via a hostRules settings ?

I think it's ok to try/fail but I prefer we only do it once per run instead of once per dependency.