All c++ based plugins have an *.xdl64 file, which is the compiled plugin, and what does the work.

Running any plugin with an xdl64 , on MacOS Catalina(or later), will give you a warning, the first time it runs.

Welcome to MACOS....

  • Firstly, when you download a zip file which contains a plugin, do not let Safari unpack it for you. Otherwise it will get quarantined( and you will think it is an empty package)
  • Secondly, once you have simply downloaded the zip(really downloaded it, as is), do not use the system unpacker. Use pretty much any third party unpacker, other than the garbage system tool that Apple has given you, otherwise, it will also quarantine the contents. 

To allow the plugin to load normally :

  • Open the Mac System Preferences – Security and Privacy, while Cinema4D is loading.
  • When a warning window pops up, like “myplugin.xdl64” cannot be opened because the developer cannot be verified, proceed as follows:
  1. Click on Cancel
  2. In the System Preferences – Security and Privacy, click "Allow Anyway". 
  3. Repeat this for all issued warnings.
  4. Close Cinema4D and run it again. This time, when you get the warnings, click "Open"
  5. The plugin will now load normally, without issuing any further warnings.

note: if you get an update for your plugin, the *.xlib file will have changed, and you will need to repeat the above process.