This ist he kind of message that you'd typically be presented with when launching an app from an unidentified developer:
"ExifTool-10.37.pkg" can't be opened because it is from an unidentified developer. Your security preferences allow installation of only apps from the Apps Store and identified developers.
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However when looking at the Security & Privacy settings, there is no option for 'unidentified developers' any more! What you do see however is a button 'Open Anyway' for the specific app you've just tried to launch.
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So to open the app you want, you actually have to try and open it first, then go to the Security & Privacy settings and click the 'Open Anyway' button and then open the app again.
After opening the app for a second time, you are then warned that it is from an unidentified developer again, but this time you are given the option to open it.
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Clicking 'Open' will now finally launch the app.
-i