Please see https://releases.xamarin.com/preview-xcode-9-beta-5-ios-11-macos-10-13-support-preview-4/ for the details about this release, including links to the downloads and the release notes.
These are early preview builds to allow Xamarin developers to start trying the new features available from Apple in Xcode 9. Any bug reports about the new Xamarin bindings are much appreciated!
File a bug report against Xamarin.iOS 10.99 if you see any suspicious behavior in this preview Xamarin.iOS version that you wish to report.
Comment in this forum thread if for example:
- You notice an error in the Release Blog post.
- You have trouble downloading one of the installer packages.
EDIT July 27: Update title and Release Blog link for preview 2.
EDIT August 03: Update title and Release Blog link for preview 3.
EDIT August 11: Update title and Release Blog link for preview 4.