My company is also looking for a air-cap solution. It really is a pain and unfortunately, the Microsoft solution that requires GitHub Enterprise isn't a good fit. That's why we'd really like to have a solution like ProGet.

As I understand it, two things are required:

1. It must be possible to configure custom service endpoints in VS Code
   --> Microsoft allows this with these settings: https://github.com/microsoft/vsmarketplace/blob/main/privatemarketplace/latest/README.md#5-connect-vs-code-to-the-private-marketplace

2. The API documentation must be available
   --> There is no official documentation from Microsoft. However, Open VSX has documented the API for its VS Code adapter here: https://open-vsx.org/swagger-ui/index.html?urls.primaryName=VSCode Adapter# The question now is whether this documentation is reliable enough for you to adapt it. The API doesn’t actually look very comprehensive. It would be great if you could look into that

Another approach would be a VS Code "Private Marketplace" extension that supports NPM registries. There are some available, but they don’t seem to be actively maintained and are therefore not very trustworthy.