I've configurated a B2C Userflow to permit login from an external IDP based on Keycloak and an Azure AD B2B configurated as OpenID IDP.
To connect the B2B, I've mapped the sub claim. As I read here, sub isn't present in User information, but I can obtain it only from token.
I need to prevent sign up for Keycloak users, but let B2B user do. With user flows I can't do in the same flow. Can I do with a B2C custom policy?