Xcode 9.1 Release was released on 31 Oct 2017 and requires macOS 10.12.6+.
It can be downloaded from https://download.developer.apple.com/Developer_Tools/Xcode_9.1/Xcode_9.1.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-SW881.
The SDKs it contains are:
- iOS 15B87
- macOS 17B41
- tvOS 15J580
- watchOS 15R844
The compilers it contains are:
- clang 9.0.0 (Build 900.0.38)
- swift 4.0.2 (Build 900.0.69.2)