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Like majority of modifiers, FFD is not supported by FBX format. You need to bake your animation before export. You can use point cache, or export to alembic format.
the purpose of exporting into fbx is use in unity , point cache will create a record animation XML script , that script can be use in unity ?
and that alembic format will be import on unity as it is with FFD animation ?
I have no idea about Unity. You have to try and see what works, or google answer by yourself.