I use Audio Filters bundled with Premiere and I also use a bunch of Audio Units plug-ins from macOS (AU) and plug-ins from different vendors (AU, VST.)
When updating Premiere, all non-Adobe plug-ins are disabled by default, which might lead to a "Audio Filter Missing" message on loading a previous Project:
I had a missing Audio Filter in my Project, showing as "Offline".
Turns out, the missing Filter is the AU Multiband Compressor, which is a macOS Audio Filter. (I still have the previous version of Premiere and I opened my original Project to see what was missing.)
With this confirmation, this is how to turn these plug-ins back on:
Click on Scan for Plug-ins, and then click Enable All (or select plug-ins individually) and Click OK.
That's it! Now all your "external" audio plug-ins will appear in Premiere Pro Audio Filters Menu.
