Google maps for iPhone is incredibly confusing when it uses "custom chrome tabs". Unlike almost every app that hands off to a browser application. When in a "custom chrome tab", I want to go back maps, so I hit the home button. Because that's how you switch apps, but wait, where is maps? Oh I'm in maps. I don't see how this could user test well when it's an anti pattern to the device itself.

Almost anything can user test well. It's really hard to get right, and really easy to fool yourself.

It is definitely not idiomatic on iOS.

Did the team think eliminating a banner or frame that wraps enclosing content was not MVP material? I'm glad Google has now made this available, but I was shocked - and unhappy - when I first realized AMP did not originally have it.

AMP had 2 multi month public developer previews. Unfortunately it did not come up as a big issue during that phase.

Since I do not stay up to date with the latest from Google all the time, and didn't even know AMP was a thing until I encountered it in a production google search one day, I had no chance to offer feedback. I imagine many iOS developers, who must be focused on native app things rather than Web-based things, would be in a similar boat; we would be happy to provide feedback, but I don't know how I would have even been asked for feedback, let alone have discovered that something like this was coming so I could proactively make a comment.

Any ideas how this process could be improved?

I hate it for custom tabs too. But what I really hate is the way AMP hijacks scrolling and page navigation. It totally breaks the feature of Android Chrome that allows you to quickly search a selected word by swiping up from the bottom. Have you tested the interaction of AMP with that prominent Chrome feature? It's symptomatic of the way AMP breaks web assumptions.