The alternative wasn't an ID requirement, the alternative was the client/OS sending the flag to the server/app.

The fear is that once you have devices sending services a flag, some asshats are going to start demanding that it be verified by the government.

But how does that do anything for you either way? Either you have two different versions based on whether the flag is present or not or you have one version and if it's adults only then you have to send the flag indicating you're an adult in order to use it.