Unlike in previous versions of Minecraft, you are no longer able to bind the fn key to an action. Though it should be noted that the function keys (f1, f2, f3, etc.) still work.
Linked issues
Comments
Erik Broes
migrated
I at least know that a friend of mine also has this problem (since we both used it as a sprint key).
Can confirm, on a mac it doesn't see this as a key-press. I think glfw might not support it but I cannot find anyone else complaining about it.