It's a "hack" because it's using the software in a way that Apple didn't intend. You could also call it "unsupported".
Not just Apple. It was included so carriers could perform tests on their network (hence "field test mode"). Neither Apple nor carriers ever intended end users to make use of it, or even be aware of it. It just sort of leaked out. Who in their right mind would enter an absurd string of characters into their phone and press Send?
And the fact is that no one really wants field test mode; they want the signal strength in DBm to be displayed instead of dots (or bars). All of the other neat features of field test mode are ignored, except by carrier technicians (and those features are still available in field test mode). So in submitting feedback that is what you should ask for; it could be added to one of the settings.
Puts me in mind of another absurd string of characters that was never intended to be used by end users: CTRL+ALT+DEL. It originated with OS programmers who wanted a way to get into debug mode in MS-DOS. They figured it was a combination that no one in their right mind would ever try.