Show as Login option for OIDC IdP as a login option on the login page not appearing
1. Make sure customer is using this feature only for OIDC and the Identity provider record is on "oidc_identity_provider" Table
2. Make sure customer is does not have domain seperation plugin installed.
Show as Login option is not supported for Domain Separated instance , this is because for specific tenant ServiceNow will never know whether that Identity Provider is intended for that tenant or not.
OIDC and Identity Provider are supported with Domain Seperation plugin. Only the Login Option is not supported with OIDC