Note: As of 8.1.31, the following steps are no longer required. The Mac Ignition installer is now notarized. With those changes, there should no longer be a need to disable the Mac Gatekeeper.
As of the release of Catalina, App Notarization is strictly enforced on macOS. Apps not notarized will not be installed or opened. This includes builds of Ignition 8.1.30 and below, as well as various launchers (Vision Client, Designer, and Perspective Workstation) from Ignition 8.1.15 and below.
The warning is due the operating system not recognizing the installer. Should you run into this scenario, you can work around it by temporarily disabling the validity check.
You can by pass this by running the following from Terminal
sudo spctl --master-disable
Once disabled, run the installer by right clicking on the
.app, and selecting "Open". After installation is finished, you can re-enable the check with the following Terminal command:
sudo spctl --master-enable