It's a right PITA - firstly I (clearly) need to do this so infrequently that I don't remember how (never a problem when we had a box to tick), and secondly the [plain] [/plain] tags aren't available as a formatting button like Bold, Italic etc, and so are much less handy to do than having a box to tick.
Another one for the list of new forum "features" which are nothing but retrograde steps that hinder, not help, users.