How does one build an LTS release ISO?

I have followed the documentation at the following URL: Building VyOS — VyOS 1.2.x (crux) documentation

I get a symlink to the artifact, but no artifact. I have tried this by setting the --version flag to values from 1.2.6 through 1.2.9. I know 1.2.6 used to work because I have an artifact from December. I doubt 1.2.8 or 1.2.9 exist.