It's not on the permitted list, so it must meet all the conditions of
422.16 (A) Flexible cord shall be permitted (1) for the connection of appliances to facilitate interchange or to prevent the transmission of noise or vibration or (2) to facilitate the removal or disconnection of appliances that are fastened in place, where fastening means and mechanical connections are specifically designed to permit ready removal for maintenance or repair and the appliance is identified for flexible cord connection.
That is a very, very, very weak case, that will only fly with Code pedants who like you, and no inspector is going to accept that.
The best way to bring a furnace (and other appliances) out to a plug connection is to add a subpanel to your house's electrical system, and place the furnace's branch circuit in that subpanel. Then (or rather, from the outset) you install a generator interlock so the subpanel can be dual fed from utility and generator. On the generator backfeed, you bring that out to an inlet, which is simply a plug inset into the wall.
You can put as many circuits as you please in this subpanel, heck if an interlock kit is available for your main panel, you can put the whole kaboodle on the interlock.
Or it can be as small as a QO 4-space panel with a QO2DTI interlock between two breakers. One must be 2-pole due to the way that interlock fits.