https://www.dailywritingtips.com/used-to-vs-use-to/
When the statement is positive, as in the reader’s example, the expression is used to.
In negative statements, the expression is use to. For example, “He didn’t use to go to the game on Friday.”

https://www.merriam-webster.com/words-at-play/is-it-used-to-or-use-to
Use was once commonly employed as an intransitive verb meaning "to be in the habit or custom".
But this sense of use now occurs ONLY IN THE PAST TENSE with to in the phrase USED TO.

Grumbler, so if I say, for instance, "I'm not used to such a treatment" it's gonna be wrong?

I think it's incorrect, but don't take my word for this. I gave a miss to this request because I wasn't sure.