There was a discussion about this before: New versions FFGL
The older FFGL plugins work in VDMX because they are apart of the old (outdated / abandoned) FFGL framework. VDMX6 runs on Metal which means that the depreciated FFGL code had to go.
More current conversations about FFGL, are because the Resolume has their own version of FFGL that they still use. https://github.com/resolume/ffgl