But I always wonder how much of the sound we hear by feeling (skin, hair vibrations). Because maybe I can't hear 1Hz, but I can very well feel it.
21-bit would make for an awkward file format. If we can hear 17 bits of range that's enough to justify storing music as 24-bit.
I have very sensitive hearing, I hear things most others cannot. I can detect 192kbps vs 320kbps mp3 encoding. I cannot have any switch-mode AC/DC transformers in my bedroom because the switching noise actually keeps me awake (I charge my phone in the kitchen, most people think I'm mad when I complain about the "noise" :)
Still, I've never been able to detect frequencies above 20Khz or hear the difference between 16 and 24 bit. I think anyone claiming to is full of crap.
24-bit is pretty much useless for playback. While human hearing range may extend past 16-bits of dynamic range, that doesn't mean that full range is musically useful. It is, however, quite useful to record and mix in 24-bit for the headroom.