The app isn't showing it because it was programed to turn off based on the 9/30 date. Tesla may or may not update the app to show it again. There's a possibility they may choose to just leave it hidden for now to prevent new orders post 9/30 from selecting FSD transfer. If you had specified the FSD transfer before 9/30, it will still be in the system on the backend.
I just asked Tesla today via my text thread with them and they confirmed that my order is still eligible for FSD transfer. Others have also confirmed this with their SA in previous comments above.
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