The German eID already has this feature. You give it a number of years, and it replies with a simple yes or no. The problem is that it's expensive to implement

The German eID already has this feature. You give it a number of years, and it replies with a simple yes or no. The problem is that it's expensive to implement. Your options are A) going with an identity provider who charges you per session, B) interfacing with an eIDserver provider yourself (which the identity provider in A does for you) with all the necessary certifications, government red tape, and UX intricacies, or C) also building your own eIDserver, which is a lot more certifications and red tape, plus mandatory very specific hardware HSM that, to my knowledge, aren't even available anymore. Add to that the users who, so far, are most likely confronted with this for the first time and the clunky "official" apps (though you could implement your own) that require your website to do funny stuff like redirecting to localhost.