This is not IRC commands we’re talking about though, this is a message to a channel that contains a command, which contains the username. Commands such as /mod or /ban appear to be intended to work case-insensitive in regards to usernames. While /w may be kind of a special command, it still appears to work the same in that regard and worked and still works case-insensitive when you send it through raw IRC.
Of course it may be safer to user lowercase anyway, but it’s only necessary for stuff like joining channels (which can sometimes be a pitfall when using a regular IRC client).