It seems as if the official RawTherapee 5.8 Linux AppImage is missing the Lensfun database (which looks as if it could be the same bug as with the initial Windows installer).
In the menu “Profiled Lens Correction”, lenses are neither automatically detected, nor is it possible to manually select a camera/lens combination.
Lensfun is installed on my system (Debian 10) via the system package manager, with the following packages:
liblensfun-bin 0.3.2-4 liblensfun-data-v1 0.3.2-4 liblensfun-dev 0.3.2-4 liblensfun1:amd64 0.3.2-4
However, this doesn’t fix the problem, and the RT 5.8 AppImage doesn’t seem to ‘see’ those libraries.