Does Xcode 9 Beta only support 10.13 High Sierra?

Doc states :

Xcode 9.0 requires a Mac running macOS Sierra 10.12.4 or later.

h t t p s : //developer.apple.com/library/content/documentation/DeveloperTools/Conceptual/WhatsNewXcode/xcode_9/xcode_9.html#//apple_ref/doc/uid/TP40004626-CH8-SW1

It also states :

If your app doesn’t require the latest OS features, you can configure it to run on a previous version of the platform operating system using the OS Deployment Target option in the Xcode Project settings. If your project was created in an older version of Xcode, you can let Xcode update your project. For details on this feature, see Project Modernization.

I don't see any step-by-step instructions here in the linked document. Can you please give them?

No. I am running it on Sierra

In Xcode 9, select the project item (at the top of the navigtor pane), then choose the General tab in the editor. Select the project in the Project listing in the editor (above the Targets list), and then choose the Deployment Target from the popup.

(You can set this on the targets individually, but setting it at the project level changes all the targets in one step, assuming you haven't previously overriden individual targets.)

If your app uses Swift, you can set the deployment target to any macOS version back to 10.9. If you don't use any Swift, you can set it back as far as you want.

Note that it no longer matters (much) what SDK is being used to build the application. It only matters what your deployment target is.

Which version of sierra were you using?

I am running the final version of Xcode 9 (Version 9.0 (9A235)) on macOS Sierra 10.12.6.