Kubuntu is and has always been “Ubuntu” combined with the greatest software from the KDE Community. Prior to 23.04 and aligned with Debian packaging, Flatpak and Snap package sources were included as installable options with each source requiring its own set of commands and repositories. However, enabling both sources caused confusion with new users (and *Ubuntu support groups) as the user was unsure about several issues including which or how their packages were installed; the proper place to report issues; and/or how to remove them. As a result, Canonical, Ubuntu’s parent company, and the Ubuntu Technical Board have chosen to support Snaps only upon initial install.
User and community member, arraybolt3, provides more details regarding challenges of Flatpak support.
No worries; Flatpaks are still available for all Ubuntu flavors. To enable Flatpaks in KDE’s Discover in Kubuntu 23.04, run this command:
sudo apt install flatpak plasma-discover-backend-flatpak
To enable the largest Flatpak repository, run this command:
flatpak remote-add --if-not-exists flathub https://flathub.org/repo/flathub.flatpakrepo
Log out and log back in (or restart) to re-initialize the XDG_DATA_DIRS variable, otherwise, newly installed Flatpak apps will not run or appear in the startup menu.