* What is your evaluation of the proposal?
+1 This makes total sense. I was going to voice my disagreement until I saw that I did not understand what the APIs did because of their names :D The renaming makes things much clearer.
* Is the problem being addressed significant enough to warrant a change to Swift?
Yes.
* Does this proposal fit well with the feel and direction of Swift?
Yes.
* If you have used other languages or libraries with a similar feature, how do you feel that this proposal compares to those?
Even if some languages have flatten, it does seem like a special case of joined.
* How much effort did you put into your review? A glance, a quick reading, or an in-depth study?
Quick read.