Can’t apps and services just start blocking Linux and start specifically demanding Windows or Mac?

In principle? Yes. It could become a complete cryptographic attestation chain.

I don't think we're right on the edge of that rn. But years or decades from now? Who TF knows.

If they can identify it, sure. But OS fingerprinting is not perfect through a browser, and can be tricked pretty readily.

Apps will get your age range from your operating system using an application programming interface (API). In other words, they’ll ask the operating system for your age range and get a response.

It's likely. But I don't know how much of an effort would be for a Linux based OS to have a pre-made token for that kind of thing.

My work programs from back when I was in accounting, uh, none of them work on Linux. They haven't even started working on Linux versions.