While most IRCDs might support it, there's a problem of them allowing users access. On Webnet for example, you are not allowed to /who an entire channel. On a lot of networks, you can't directly /who a nickname who is set to invisible.
For instance, on DALnet, you have to specify a channel when you want the WHO command to work successfully. That requires certain parameters added to the WHO command
Anyway, my point is that the usage and access of WHO varies a lot more on the different IRC networks than /whois.