I can't find the iOS 10.3 API diffs. I tried searching and also replacing the os version in the url of older API diff documents following the same pattern but was unsuccessful.
Where can I find the iOS 10.3 API diff document?
I guess they do
https://developer.apple.com/library/prerelease/content/releasenotes/General/iOS10APIDiffs/ (note the "prerelease" in the url)
This seems to show entire docs with changes highlife https://developer.apple.com/reference/uikit?changes=latest_minor
There no longer seems to be a way to get just a list of the changes. You will have to browse the new API Reference with highlights turned on.