The truth comes out... it's all about censorship of the masses:

" The EU says a new age-verification app is technically ready and could let users prove they are old enough to access restricted online content without revealing their identity or personal data. Deutsche Welle reports:

Once released, users will be able to download the app from an app store and set it up using proof of identity, such as a passport or national ID card. They can then use it to confirm they are above a certain age when accessing restricted content

...

The app is expected to support enforcement of the bloc's Digital Services Act, which aims to better regulate online platforms. This includes restricting access to content such as pornography, gambling and alcohol-related services. "

And will there be source code available for this app? Will it work on GNU/Linux? Will it work on FreeBSD? Will it work on OpenBSD? Will it work on VxWorks? Will it work on NetBSD? Will it work on a yet-to-exist operating system????

We already know the answer to this without looking into it. The answer is no. UK GNU/Linux users are already unable to access half the internet now thanks to sites only supporting a subset of the most popular operating systems, browsers, and similar. If your 'unknown' (since GNU/Linux has thousands of versions and releases alone) the default is to block you.

VPNs are more critical today than ever before as a consequence of dumb legislation like this.

You might try and arrest your way into it working by going after developers who don't comply as the US is trying to do with similar legislation and has done even where developers clearly broke no law (cryptocurrency wallet cases like Samourai Wallet).

It won't work. Development will go elsewhere and underground.