FeathersJS uses a package called Grant for most of the OAuth functionality. Grant now supports the Apple OAuth flow, but only in recent versions. FeathersJS v5 (currently in prerelease) uses this new Apple-supporting version of Grant. For this reason, I would suggest first upgrading to FeathersJS v5 (Dove) https://dove.docs.feathersjs.com/guides/migrating.html
Sign in with Apple does NOT work with servers set up as localhost. If you haven't already, you'll need to pick a domain for your FeatherJS server. If your main domain is example.com, you might want to pick a subdomain like devapi.example.com. You won't need to change the domain's real dns for this, you can add the alias to your /etc/hosts file:
