This article says that in American English it’s considered an error, but it’s accepted in British English
I think that “used to” is the form seen so often that “use to” just looks and sounds weird to me. I would never use it
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-...