pearos-liquidgel can be left behind by a regular update if it needs to be rebuilt against the new system. The pearOS Welcome app has a dedicated button for this.
Open the pearOS Welcome app and click Fix Liquid Gel.
This runs the same update as Update System, then reinstalls pearos-liquidgel in the same root session - so you only get asked for your password once.
After it finishes, log out and back in (or restart plasmashell) for the slider to come back.
On a fresh pearOS Goldwing (Debian) install: Liquid Gel is fetched during the first-boot setup wizard, and only if you have internet access at that point - if you set up your account offline, Liquid Gel is simply skipped rather than failing loudly. Connect to WiFi/ethernet and use Fix Liquid Gel in the Welcome app afterwards to pull it in.