The “/” check seems to happen after the message is filtered, so commands still display if the first word is censored.
My testing place has a “/morph” command that randomizes your character:
Not related to filter, but related to chat commands: Typing /sc in the chat will make your character say “ROOT” and if /sc is proceeded by a number you’ll say the old menu chat stuff.
Currently chat is expected to be hidden when preceded by “/” and the filter changes that here. Not a serious bug, but it can be a tad annoying.