Possibly mTLS, which you’d configure in your reverse proxy. You could email them the certificate and instructions on installing it. I believe for Chromium browsers on Windows you basically just double click the cert and click through the wizard. Firefox I know has a thing in the settings for importing the cert. Android you just tap on the cert and make sure it opens with ‘Certificate Installer’ if it gives you the option.

I found a post on the forum:

https://forums.truenas.com/t/scale-build-git-repo-going-closed-source/64313

This is only their old build system which they weren’t using themselves, the rest of the OS will remain open source. However they also said some worrying stuff about including “proprietary pieces of the OS”.