In previous versions of Xcode, we could hold down Alt/Option key, drag the mouse to select columns in the text editor. In Xcode 9 Beta this doesn't seem to work.
Is this gone now, or is it a different key combination? Any ideas?
According to the Xcode 9 release notes it is a missing feature. I can only assume it will be added back in before the official release.
From the Xcode 9 Release Notes (page 21):
Xcode 9’s new source editor does not yet support discontiguous or columnar selection. (30738532)