It looks like the <https://github.com/commanderx16> github account no longer hosts the the Commander x16 emulator source code or the source code for its ROM. These repositories have been migrated to <https://github.com/mist64> instead. As a result, the PKGBUILD isn't going to work anymore until it is updated to point to the new repository.
The documentation and demo code is still hosted at <https://github.com/commanderx16>, so my hunch is that the repository owners are merely separating concerns.
Pinned Comments
Juju commented on 2020-02-09 04:46 (UTC)
Good news everyone: the X16 team upstream just announced they have a preliminary agreement with Cloanto, which means they don't have to rewrite the KERNAL (aka x16-rom) from scratch, and the repos are officially back online! So I'm going to revert the PKGBUILDs as they were before real soon.
Give a huge thanks to Perifractic who made this possible (you can find him on YouTube), and vote for the packages if you like them :)