Running pre-built qemu-system-ppc (such as the one we provide) in Catalina has become problematic: Apple requires code-signing while Qemu is not built with the default Apple developer tools (it uses parts from brew). To not require users to install brew, we also redirect Qemu-system-ppc to use the libraries in the Libs folder. 

To by-pass Apple's code-signing requirement for the Qemu application you can run the allow_qemu_to_run.command script. You can check what the script does by opening it in e.g., textedit: It removes an attribute from the files mentioned in the script. This attribute is used by Gatekeeper to notice whether a file is signed/notarized/downloaded.

Select the folder you downloaded and use right-click (or control-click) to select the option "New Terminal at Folder".

Next, run the allow_qemu_to_run.command as root: 
sudo ./allow_qemu_to_run.command

You can now run Qemu by double-clicking the qemu.command file (after editing it to suite your situation) without getting warnings. 