IDK. I don't think we can actually have a discussion about education where all statements are supported by indisputable evidence.

There do happen to be citations for this question but I doubt any really clears an "indisputable evidence" standard. That's the nature of the field. Even if the whole discussion was evidence based and dotted with citations, we'd still be working with a lot of intuition and speculation.

Not a citation, but I believe it's a mediocritizing measure. For some teachers and some students, teaching to the test is probably better. I suspect more heavily concentrated in the bottom 50% of each group. For a subset of great teachers and great students, it's a detriment.

I saw some professors share the least about their tests to make sure we truly understand the material, sounds to me like a real-life usage of a train/test split. It’s not far fetched to think they employed this technique because teaching to the test didn’t work well by itself.