SoundHack VST, VST3 and Audio Unit plugins are now compatible with macOS Catalina. AAX plugins have yet to be tested thoroughly. When updating to the new Catalina plugins, make sure to delete the current plugins from /Library/Audio/Plug-ins first.
The classic SoundHack program from 1991 is not compatible with macOS Catalina. It was programmed initially on Mac OS 7, and subsequently updated with the CarbonLib to run on OS X and then macOS. CarbonLib is only available as 32-bit, so SoundHack cannot be updated to 64-bit. A replacement program is in process, but not everything in classic SoundHack will be recreated.