Boils down to some files in the VST2/3 SDK.
aeffect.h and aeffectx.h
which are not found in the path. Note that the JUCE options to set paths etc. have changed since most of the online materials I have found were published. It is no longer possible to set a VST3 SDK path. Although I read that VST3 SDK is included in JUCE now I downloaded and installed it anyway.
I found files audioeffect.h and audioeffectx.h and tried copying those to the similarly named files the code is looking for, modifying the juce_VST_wrapper.cpp file to point directly at those using the full path, but it does not appear to be the solution.
I tried following this but as I said, the UI no longer offers some of these options.
I’m using the Faust IDE and exporting the code as juce/plug-in.
Projucer is 6.0.8.
I’m able to successfully build the Plugin examples from JUCE, e.g. the Surround Sound VST3 works fine without any fiddling around.