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I found versioning stable currently confusing as we have two versions of
Turris OS. It should said Turris OS 5.x HBS, because the stable version
of someone can mean that he could download stable branch on our
repo.turris.cz and try that version, which is not correct, though.

While at it, I also noticed that the LXC container for Turris MOX
regarding Turris OS was missing, so add it as well.

The latest version of Sabayon is currently 19 and I could not find any
mention about ARM release and it is not provided on linuxcontainers.org

So, let's drop it. It is not recommended to use old and outdated OS.

End of life for Standard supports for Xenial Xenus ends in April 2021.

The previous URL shows 403.

mail_notifier was moved to turris/user-notify> about an year ago