I have been using Isotope noise reduction software as a plugin with FCPX for more than a few years. Updating to Catalina and the plug no longer shows in FCPX, and their site informs to use offline and save a wave file.... Round robin!
It was likely a 32-bit plugin; Catalina only supports 64-bit plugins.
To be honest, I never use the RX plugins when using Final Cut or Resolve; they are very processor-intensive and lack many of the features of the full RX application. In Resolve you can set RX as an external editor, which is a great feature because it allows you to use the full RX app without having to export any files; Resolve creates a copy of your original audio file and sends to RX, and the changes you make to RX are saved to that file; you always have the original as a backup. It's a seamless operation, like having RX available within Resolve. It would be great if FCPX could do something like that; even the Reaper DAW has this feature.