Xcode 9 support on macOS Mojave

Yes.

You can use Xcode 9 (last release is Xcode 9.4.1) on macOS Mojave.

If you are upgrading from macOS High Sierra, you can choose to not update your copy of Xcode installed from the Mac App Store.

If you have already upgrade your App Store installed copy of Xcode to Xcode 10, you can download Xcode 9.4.1 DMG from Apple Developer Downloads.

Once downloaded, double click to mount the DMG, copy Xcode.app from the disk image to Desktop, rename it to Xcode 9.app and move it to /Applications directory. You can also add drag the Xcode 9.app file to Dock to add a shortcut.

Note: I’m not sure about expect version number compatibility, thus recommended to install the last release of Xcode 9, i.e. Xcode 9.4.1.

As mentioned in your comment, you currently have Xcode 9 installed from the Mac App Store and a GM build of Xcode 10 downloaded from Apple Developer. You can now take either one of the below mentioned approaches. However, before proceeding, consider deleting the Xcode_10.app from /Applications as it is a GM build and the public release for the same is now available:

1. Leave Xcode.app (Xcode 9) as is in the /Applications directory. Mac App Store will show that an update is available. Do not install the update from the Mac App Store. However, make sure that the version of Xcode 9 is the last released version, i.e. Xcode 9.4.1. Download Xcode 10 public build from Apple Developer Download using this link. Copy the Xcode.app from the downloaded DMG to ~/Desktop, rename it to Xcode 10.app and move it to /Applications directory.

2. Second alternative would be to update the Mac App Store installed copy to Xcode 10 and download Xcode 9.4.1 instead from Apple Developer Downloads. Similarly, rename the downloaded file to Xcode 9 and move it to /Applications directory.