KUBUNTU KDE VERSION

Trisquel 9 is based on Ubuntu 18.04 Bionic Beaver and https://launchpad.net/~kubuntu-ppa/+archive/ubuntu/ppa?field.series_filter=bionic does not apparntly allow to install a more recent KDE Plasma.

If you install Trisquel 10 (http://cdbuilds.trisquel.org/nabia/ hosts the ISOs), which is not production-ready!, you can add "deb http://ppa.launchpad.net/kubuntu-ppa/ppa/ubuntu focal main" to your software sources, for instance executing the following command in a terminal:
echo 'deb http://ppa.launchpad.net/kubuntu-ppa/ppa/ubuntu focal main' | sudo tee -a /etc/apt/sources.list
(Warning: do not for get the "-a"!)

After updating and upgrading, you would not get the latest KDE Plasma (probably impossible to install on Trisquel 10, based on Ubuntu 20.04 Focal Fossa) but the versions at the bottom of https://launchpad.net/~kubuntu-ppa/+archive/ubuntu/ppa?field.series_filter=focal

In Trisquel, https://gitlab.trisquel.org/trisquel/package-helpers/-/blob/etiona/helpers/make-plasma-discover "disables kns backend" because "store.kde.org allows cc-noncomercial". There may be other freedom issues. It is even possible that the PPA introduces or will introduce issues absent from upstream. I am not only referring to freedom issues: it is technically possible that the PPA contains malware. In the end, if you use the PPA without checking the related source, you need to trust those who administrate it.

Install pre-release Triskel 10 to see which version of Plasma it ships.

For your freedom's sake, use Kubuntu's PPA with extreme care.