Whether or not there is an approved spelling, this looks like a case of the "ice cream" phenomenon, where a final dental stop (/d/ or /t/) gets deleted before a consonant-initial word.  Other examples are

skim milk
ice tea
stuff peppers
etc.

The two spellings would probably be pronounced the same, without the final /d/.

Herb
From: Assembly for the Teaching of English Grammar [mailto:[log in to unmask]] On Behalf Of Dick Veit
Sent: Tuesday, June 14, 2011 10:45 AM
To: [log in to unmask]
Subject: didn't use(d) to

Two quotations from recently encountered novels:

"There's bad blood now. Didn't use to be like that..." (dialog in Alan Furst's Spies of the Balkans, p. 102, Kindle edition).
"She didn't used to smoke around the kids..." (Kate Atkinson, When Will There Be Good News?, p. 126, Kindle edition).

So which is it, didn't use to or didn't used to?

A few usage guides I consulted prescribe "didn't use to," but others say both are standard. In my own writing, I probably would have used "didn't used to."

On the one hand, "used to/didn't use to" would parallel other verbs (laughed/didn't laugh), but, on the other, we're talking about a quasimodal, and with modals we can expect significant variations from other verbs. Pronunciation is no help--both "use to" and "used to" are spoken identically as "useta."

Thoughts?
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