Xcode 9.0 Release was released on 12 Sep 2017 and requires macOS 10.12.6+.
It can be downloaded from https://download.developer.apple.com/Developer_Tools/Xcode_9/Xcode_9.xip.
Release notes are located at https://developer.apple.com/library/content/releasenotes/DeveloperTools/RN-Xcode/Chapters/Introduction.html#//apple_ref/doc/uid/TP40001051-CH1-SW878.
The SDKs it contains are:
- iOS 15A372
- macOS 17A360
- tvOS 15J380
- watchOS 15R372
The compilers it contains are:
- clang 9.0.0 (Build 900.0.37)
- swift 4.0 (Build 900.0.65)