This year’s macOS update is out, and with it comes increased focus on protections against malicious software. The latest unstable build of 2.2 (#3209) now complies with the new requirements. I’m also looking into making an updated build of the stable 2.1 for macOS.
The new requirement is that applications must be pre-scanned by Apple (“notarized”) to ensure they are properly signed and contain no malware. It took a few builds to get this working right, as it turns out I was using quite an old version of the SDL2 frameworks on the macOS build machine and those were not being signed correctly.
Another issue you may encounter now with Catalina (and Mojave) is that looking around using the mouse/trackpad causes the player view to just spin around rapidly. This is because Doomsday’s mouse input relies on being able to reposition the mouse cursor while it is hidden. If an application wants to change the cursor position, this requires that the user has allowed it to control the computer in the Security & Privacy > Accessibility system preferences pane. If you encounter this problem, drag and drop Doomsday.app from the Finder to the list of apps on that pane.