Not quite true. Handling WiFi changed in 5, and changed again in 6 (really only relevant if your app deals with changing the Wifi state, admittedly). 6 introduced Doze, which can require different implementations of things. There were a number of graphical things in 5 which, if you wanted them on earlier versions (and your Design department does, and you're going to do it, dammit), require fairly different implementations.