Monica's answer addresses the point about understanding what may be confidential and how to ask, but I'd like to suggest that you can get useful information without asking to see a document that may or may not even exist, let along dealing with its possible confidential status.
You indicate that one of the reasons for wanting to see this information is because in your experience the artifact of the sprint retrospective can show "what developers love and hate about the last few weeks". But could you not also simply ask in the interview for a summary of some of the common/frequent things that the development team wants to a) stop doing, b) start doing, or c) continue doing well?
For me as a hiring manager, a question of that type (rather than just "can I see your retrospective artifacts") would indicate to me that you respect potentially confidential internal information (again, Monica's answer also describes a way to do that), understand the typical goals of a sprint retrospective, and are comfortable having a conversation to elicit and discuss information rather than reading and interpreting something on your own. That last one might not be meaningful to you or them, but it's something that I value and I don't think I'm that weird: being able to ask for information and have an ongoing dynamic/organic conversation about it rather than just reading something and saying "ok, thanks" or whatever.
Another reason for asking for a summary or interpretation of common retrospective outcomes rather than an artifact is because not all groups produce artifacts. On my teams, our public artifacts are extremely minimal if present at all, and we most certainly don't record them, because typically these are the "safe spaces" where the team talks about issues meaningful to them, sorts them out, and has to assume that they're in a "circle of trust" in order to be open and bring about change. If we recorded them or took detailed notes, then it would be just another public activity, and I've found that not to be the best approach -- but again, that's me and my teams, and everyone is different.
In other words, as a hiring manager I would balk at the request to see internal documents or raw data, but if the question were phrased appropriately I would be happy to discuss in general ways anything that would be helpful for a candidate to judge the company and the team. So, I would recommend thinking about the information you're trying to elicit and how you might do in the typical course of an interview (especially during the important "Do you have any questions for us?" time).