The "Require email verification" is unavailable for custom issuers, as the code explicitely enables it only for well-known issuers like Google or Facebook.
We use our own OIDC provider in our instution, and all services require login through it. In our case, it makes no sense for an user to exist outside this OIDC provider.
As such, we would like this button to be available in the edition of our issuer.
In addition, the OIDC specification provides a "email_verified" standard claim, indicating that the provider verified this end user's email. It would be interesting to skip the email verification phase if such a claim is found.