Did a manual download of the 100.6 install .pkg this morning to use with the AR Sample project and got the following:

FYI if you go into Settings->Security & Privacy->General it will show an option to open anyway. once you chose that option you will get a new version of this popup with an "Open" option.
This whole untrusted developer workflow seems to have changed a bit with Catalina and threw me for a bit of a loop. I generally install via CocoaPods so this might be par for the course and I'm just out of the loop?