According to this (fairly old, but still applicable) topic: New plasma renderer notification • KDE Community Forums, you should set it to “automatic”. My AMD system has it set to “Automatic” for the rendering backend & render loop, and “force” is unchecked for GL Core Profile.
The thread mentions that the software rendering message is most likely to appear on Nvidia desktops. When the driver breaks after an upgrade or something, Plasma falls back to software rendering to keep the desktop working.
You can adjust the renderer settings by launching plasma renderer from Krunner.