The checkbox "Use F1, F2, etc. keys as standard function keys" in System Preferences » Keyboard is replaced by Touch Bar options but my MacBook Pro has no Touch Bar. defaults write experiments I've tried did not work.
Workaround: Fluor App. Even better: enables fn invert only for selected apps.
Mac model: MacBook Pro (Retina, 15-inch, Mid 2015)
Mac version (optional): 10.15.3 (19D76)