Versions in the distributions
Some Linux distributions like Ubuntu and Debian could offer Remmina Plugin UltraVNC in their repositories but very often is an old version. The release process for a software to reach the official repositories in the distributions is difficult and requires a lot of time.
The Latest available version is that one indicated in the releases page on GitHub.
The following are packages for different distributions. Choose that more suitable for the distribution in use and refer to the Installation page for information about their installation.
Version 18.104.22.168 - Release date: 2019-12-21
Version 22.214.171.124 - Release date: 2013-05-27