As stated in this github issue, claims_supported is mutable. Values are : "aud", "sub", "iss", "auth_time", "name", "given_name", "family_name", "preferred_username" ,"email", "acr". Admin user can only hide claims_supported.
It is a common use case configured Keycloak to support more claims than the default.
So we have created a PR in order to admin user being able to configure claims_supported.
We have added separate tab in realm settings for configuring claims supported. If you believe that we can put this configuration inside other realm settings tab, I could change it.

Could you review this PR?