These errors are generated on a clean build:

C:\cpython-3.8.5\Modules\_ctypes\_ctypes.c(107,10): fatal error C1083: Cannot open include file: 'ffi.h': No such file or directory [C:\cpython-3.8.5\PCbuild\_ctypes.vcxproj]
C:\cpython-3.8.5\Modules\_ctypes\callbacks.c(4,10): fatal error C1083: Cannot open include file: 'ffi.h': No such file or directory [C:\cpython-3.8.5\PCbuild\_ctypes.vcxproj]
C:\cpython-3.8.5\Modules\_ctypes\callproc.c(71,10): fatal error C1083: Cannot open include file: 'ffi.h': No such file or directory [C:\cpython-3.8.5\PCbuild\_ctypes.vcxproj]
C:\cpython-3.8.5\Modules\_ctypes\cfield.c(3,10): fatal error C1083: Cannot open include file: 'ffi.h': No such file or directory [C:\cpython-3.8.5\PCbuild\_ctypes.vcxproj]
C:\cpython-3.8.5\Modules\_ctypes\malloc_closure.c(2,10): fatal error C1083: Cannot open include file: 'ffi.h': No such file or directory [C:\cpython-3.8.5\PCbuild\_ctypes.vcxproj]
C:\cpython-3.8.5\Modules\_ctypes\stgdict.c(2,10): fatal error C1083: Cannot open include file: 'ffi.h': No such file or directory [C:\cpython-3.8.5\PCbuild\_ctypes.vcxproj]

Yes, if you use -E then you need to provide the dependencies in some other way.

The option is there to suppress network access at build time, not to build without those modules.

The documentation states:
  -E  Don't fetch or build external libraries.  Extension modules that
      depend on external libraries will not attempt to build if this flag
      is present; -e is also accepted to explicitly enable fetching and
      building externals.

So, according to this, any module that is dependent on external libraries will not be built. Ant it was working with 3.7.5.