If that's proof, I'm writing from an iskeai with internet access. I do agree that she regressed mentally, but this is no proof.
If you want to know how you prove that a character regressed mentally see https://mangadex.org/title/51376/reincarnated-as-an-aristocrat-with-an-appraisal-skill chapter 5. There is no way an adult would say that, or act like in that way. And those actions are the main point of the chapter, in the main story.
This is a proof only if you already believed that to be true, as this chapter could be explained also with the opposite theory. I've seen plenty isekai with adult protagonists that are treated like this to break the tension, and it happens only because they are misunderstood.
For example:
- "Don't run off on your own" is something you say to kids, and has nothing to do with their real mental age. If anything that's just her not adapting to others seeing/treating her as a kid.
- Running out onto the road is the same thing. For example an adult can can check the road with peripheral vision, sound, or by only moving the eyes. She is an assassin, I think she'd be able to do it. If a kid did this you'd think they didn't check, because that's how kids usually act.
- Her reaction to her maid stopping her would be natural too, as she didn't do anything wrong for an adult.
- This is not in the main story. Plenty of authors misrepresent characters or make them behave a bit out of character, just to be more cute or make the audience smile. You can't prove her mental age based on a 4 koma extra. Not to mention that all this could have been in the maid's point of view, which may be unreliable.