Apple now requires developers who use 3rd party sign-in options, such as 'Sign-in with Google', to also include 'Sign in with Apple'. There are some exceptions but only a handful. You can check them out on section 4.8 Sign in with Apple in Apple's AppStore guidelines.
Just let that sink in for a bit. Soon we all might be logging into all our apps with Apple Sign-In. All our apps! Seems like Apple might become a normal consumer's Single Sign-On (SSO) system. SSO provides the ability for people to login to one system, which then grants them access to multiple apps, services, or sites. Many corporations have a SSO system, either it's an internally developed one or they are using a third party service like Okta.