I myself bumped into an issue when using pnpm and testing with npm, as in here. Subsequently I discovered the discussion on npm here which seems to suggest
file: is soft-deprecated, and
link: is now supported. Yet, in practice (tested with npm 5.6.0, 6.4.0), the opposite seems to be true.
npm install works fine for
file:, but throws a
npm ERR! code EUNSUPPORTEDPROTOCOL npm ERR! Unsupported URL Type "link:": link:X
I was unable to find official documentation on the subject.