I’m sorry I misunderstood this as script instead of a filter with its UI. The issue is that this is a new LADSPA plugin, and the MLT in Shotcut does not read LADSPA plugins installed to locations other than the Shotcut install. OTOH, it does read VST2 and LV2 from the system and user-supplied paths:
It you put things into the Shotcut install dir, it has a good change of being removed on an update or reinstall. And some installs do not allow modifying the Shotcut program folder (Microsoft Store app, macOS, Linux AppImage/Snap/Flatpak).