Over at the English Language Learners [0] Stackexchange, reading through the answers and how they describe their references (I didn't click through to read those references), it seems like "didn't used to" is more acceptable in general for both American and British English, even though it shouldn't be correct because it breaks the rules.
I like this last part of one of the answers in particular:
> One of the strongest arguments against treating used to as a standard verb form is the pronunciation. According to the Cambridge Dictionary, with the verb form of use, the s is voiced /juːz/ whereas in the noun form it is not voiced /juːs/. In I used to and I didn't used to, the s is unvoiced. It's definitely not a noun, but it is also not a standard verb form: it is something unique, and that's what upsets the grammarians.
Aside, like sibling comment, I'm also a native American English speaker and it's always been "didn't used to".
[0] https://ell.stackexchange.com/questions/100674/didnt-use-to-...