Kubuntu is one of the oldest, official community flavor of Ubuntu featuring modern Plasma desktop. It brings plasma desktop with a decent customization such as artwork to make it more appealing to end users.
Like any other official community flavor of Ubuntu, Kubuntu also releases a new version on every 6 months. This release will come a relatively new and stable release of Plasma desktop available at that time and it will be the main version throughout the life span of release. However, users can also choose to enable Kubuntu backport repository to enjoy latest release of Plasma desktop.