I used not to play football.

<When using "used to" to mean, "did something in the past often," never ever split the "used" and the "to." >

Where did that "rule" come from? Isn't it prescriptive?

<It's just that "used to" is a special construction with special rules that doesn't really match the rest of the language. >

The dominant rule is 'use "use to" spelling in negatives and questions'.

<Consider "have to," "has to," and "had to." In the first two, the final consonant in the verb is pronounced soft, like an "f" and a soft "s," but in the last one, the final consonant, the "d," is still pronounced hard, like a "d." >

And what has that got to do with a discussion on the use of "used to"?

It's explaining the fact that used to and use to sound identical to most people, which probably explains why they are often confused.

<<but in the last one, the final consonant, the "d," is still pronounced hard, like a "d." > >

You do know why it is pronounced with a "d", don't you?