I know Release-System is no longer offered as an installation package, but available at
However, https://docs.joomla.org/Deploying_an_Update_Server is still mentioning it and pointing to akeebabakup.com.
Also, https://www.akeebabackup.com/home/news/1662-software-availability-changes-march-2016.html (as linked to in "#24876 – ARS Joomla 3"), in turn links to https://www.akeebabackup.com/release-system which is a 404 – should probably link to the github repo?
Also, if not a package, a link to the github repo would have been helpful at https://www.akeebabackup.com/download/developer-releases.html
(but why not provide an unsupported package as a Developer Release?)
Just as a heads-up – thank you for your software!