Are you definitely using the very latest version of JUCE? I just tried building the DSPModulePluginDemo, using the Projucer’s Xcode and VS2019 exporters. I added JucePlugin_AAX_Chunk_Identifier='blah' to the preprocessor definitions of both exporters. The builds completed successfully after that. Putting a message pragma in the AAX_Wrapper.cpp shows that the value set in the Projucer is definitely visible.
I’m using latest Projucer on Mac I had to specify it as JucePlugin_AAX_Chunk_Identifier='\VSTC\' and on Windows as JucePlugin_AAX_Chunk_Identifier='VSTC'