I don't have one yet unfortunately, I am trying to make OpenWISP play well with this type of configuration which I see has been introduced in OpenWrt 21.
AFAIK, currently, no.
Unless it's some kind of typo, but probably a feature of the sw behind the parameters.
Not reflected in any currently supported hw.
There are 60ghz radios, if you google it, but they appear to be point to point solutions.
There's also a build for the MikroTik LHGG60 (ipq40xx target-- much different from the 2.4 and 5 GHz LHG models). I have some Cube 60s which look to be very similar hardware. So I tried the LHGG60 initramfs on one of my Cubes-- it did boot up and give SSH access, but no wifi at all was configured. I didn't investigate further, mostly I was wanting a backup of the flash with stock firmware.
It's not just for me, I maintain a project called OpenWISP which allows to manage OpenWrt based devices. We are working on adding support to the new "band" syntax of OpenWrt and I want to implement this properly if possible also for 60 GH, with the hope that will be useful to OpenWrt and OpenWISP users. I also do not own a WiFI6 device yet, but OpenWISP supports this syntax because it's well documented and other users are already using WiFi6.