For many, additional modifier keys means keyboard firmware changes with macro programming. My keyboard does not have media keys, but I have programmed the keyboard that Fn+Q sends the Volume Up key, Fn+A sends Volume down, Fn+W sends Play, etc. There is no Fn key in the HID specification (at least not that I'm aware of), the Fn key exists only on the keyboard's firmware.

Many keyboards these days are getting quite fancy. There's even an open source keyboard firmware standard. https://qmk.fm/

I use X11 and a custom keyboard firmware: both of my hyper keys generate USB left super and both of my super keys generate USB right super (or vice versa), and X11 turns left super into hyper and right super into super.