RackTables 0.21.4 has been available for about a week, the main purpose of this
release is to include MySQL 8 support.
The next version will likely be called 0.22.0 to indicate the change of the
minimum PHP version -- it will be PHP 7.0 to install and PHP 7.1 to run the
unit tests. If you are interested in the technical grounds of this, please see
the related mailing list discussion from July 2019 and recent commit messages
in the git master branch. In practice this means, for example, that Ubuntu
16.04 with PHP 7.0 will probably work for a little while longer, but you should
consider upgrading soon.
For your convenience the main page of wiki.racktables.org now shows an updated
roadmap for 2019-2020.