So pin 23 generates a 32KHz square wave while the unit is powered, it is available as long as the unit is in ULPM (so effectively off). So if you are using the unit in sleep mode (at+ksleep) it will still be there, the USB_SS is not really a ‘mode’ of the unit as this just applies to the USB bus, not the unit as a whole. If you don’t have USB connected (like I have tested it with) then USB is not part of the discussion.
Does that answer the question?
Regards
Matt