It's currently possible to disable registration for an auth provider, but not possible to disable login.
Installs may reasonably want to allow linking to a provider like Asana, but require login through a narrower subset of providers (e.g., Google) -- for example, because Google has mandatory MFA but Asana does not.
It should be straightforward to support this, there was just no product motivation to implement it originally because there weren't any integration-focused use cases yet.