Why not have some sort of VCS provider do something similar to Medium/Substack/etc. where access is generally free but frequently prompts you to subscribe/donate before access?
Because tooling would route around it immediately. Among other reasons, you can't have interactive prompts for support gumming up your CI (and if they're not interactive, then they're trivial to ignore). NPM does something close to this, but it's much more opt-in for the user.