Apple's Fn key isn't ideal. The USB HID usage ID is not one from the keyboard or consumer key pages. It's in one of the "vendor defined" pages, meaning that every keyboard driver supporting has to specifically recognize the device vendor and model, because without that context one cannot know what a vendor-defined ID means, and every new keyboard supporting this requires an operating system device driver update across many operating systems as well as system firmware updates to machines whose firmwares recognize this vendor-defined ID as a keyboard key, all to add another vendor ID in to the drivers' lists of "this vendor+device has Apple's Fn key".
Still, Das Keyboard has been making a Mac-specific keyboard for many years now, you'd think they'd at least reach out to Apple to see if they can get their keyboard recognized as having a Fn key (or, alternatively, provide a kext that adds support themselves). But the one time I asked them about it, they didn't seem to even care about how the Fn key behaves, so I doubt they've even made an attempt.
* http://lxr.free-electrons.com/source/drivers/hid/hid-apple.c...
* https://bugs.launchpad.net/ubuntu/+source/linux/+bug/942184