I believe we were once experimenting with a user setting to turn off the F&R icons. Reading an ini pref from the SettingsStore is a much quicker/easier test than coding environment variable logic (not to mention coding UI integration). So it can probably more accurately be described as an abandoned/incomplete/vestigial/forgotten experiment, rather than a
secret parameter. There are no secrets in open source code, after all.
I believe it was determined that the qss option was sufficient for the 8 people in the world who refused to accept/embrace the new F&R icons at the time. No full ui-configurable parameter required.
By the way... is "Settings Catalog" really one of the translations of "Open Preferences Location"?