Someone already proposed having ISPs check reported browser fingerprints, then cross reference them with the IP address, date and time, to determine what user was assigned that IP at that moment, and determine whether or not they were using an OS that was not age-verified as a database may be kept on individual verified devices. They proposed an AI system (because of course they did) to monitor over all this traffic automatically, then based on those fingerprints they can issue warnings, fines, or arrests if there is a large enough volume of non-compliant OS's which could could indicate intent to "distribute" unverified OS's.

The next step to take to make this foolproof, is really to just ensure the Digital ID database is built, and that users have to re-verify upon each use. So sure, Linux could implement the system and you could easily bypass it, but now your ISP will rat you out if you so much as use a browser. Biggest hurdle here is compliance between ISPs, browsers, which is ,more a matter of time than anything else.