This does not appear to be available on the iPhone SE (2016). Interestingly, searching for "Back Tap" does show a singel result for Accessibility -> Touch, but once you're there the option is hidden.
I believe it requires the Taptic Engine, which only appeared on the 7 and later models.
Why is that? Isn't the taptic engine just a miniaturized vibrating unit?
I guess a vibrating unit is an electric motor fundamentally. And an electric motor is also a generator when run in reverse. Maybe the Taptic Engine can also sense vibrations too.