Hi, I made an AudioProcessorUndoAttachment class to connect an AudioProcessor with an UndoManager:
https://gist.github.com/yairchu/0a70e553af6176b2f6af
Everyone's welcome to use it, review it, report bugs, or even incorporate it into JUCE :)
It's intended for plugins that do not use AudioProcessorValueTreeState (which already has UndoManager support).
Cheers, Yair