Terminal => defaults write com.apple.sidecar.display allowAllDevices -bool YESI found this video
But it’s still possible.Terminal => defaults write com.apple.sidecar.display allowAllDevices -bool YES
It's not supported by Apple
For now. Wouldn’t be surprised if Apple removes this ability from the release version of Catalina like they removed eGPU compatibility from Thunderbolt-2 equipped Macs last year.But it’s still possible.