In macOS Catalina (10.15) and above (see this reference to the macOS versions on Wikipedia), Apple has placed significant restrictions on which installers will run without receiving a warning message. This message indicates that Apple cannot check the installer for malicious content. This will likely occur on many programs not downloaded from the Apple App store, and not related to any changes that we have made in our programs or in our installers.
In order to run the installer, double click on the Scaffold DMG file. When the dialog box opens, right click/control click on the installer app and select Open. A dialog box (image 1, below left) will open with the same warning about the installer app not being checked for malicious content. Select OK and proceed through the installer as normal.
In newer versions of macOS (13 and 14), it might be necessary double click the DMG, then choose Show in Finder, then double click again or right click/control click on the installer app and select Open. At this point, the option to Open will be available and installation can continue (image 2, below right).
If you encounter additional problems running Scaffold installers, Gatekeeper security protocols might be blocking installation:
Run these commands to disable this security feature:
sudo spctl --master-disable
Eject/unmount the Scaffold installer and try again. If you want to re-enable this security feature, run the following command:
sudo spctl --master-enable
Finally, you might also need to allow applications downloaded from identified developers in the system settings: System Settings > Privacy & Security > Security > App Store and identified developers
2023-03-28: This has been tested on Ventura (macOS 13.3)
2023-11-14: This has been tested on Sonoma (macOS 14.1)