I used Focus Follows Selection in Xcode 8 and loved it.
I cannot find it anymore in Xcode 9 beta 5... When I search Xcode 9 help gives me this result: (where it actually was inside of Xcode 8)
You can focus attention on the code you’re currently editing, easily identifying where it begins and ends, by highlighting the current scope in the source editor.
 Click here to see Apple Help screenshot
- Click anywhere in the source editor.
- Choose Editor > Code Folding > Focus Follows Selection.
But the option seems to have moved or been removed.
It would greatly help me if anyone would know how to activate it inside of Xcode 9 beta 5?
They rewrote the editor. Try
cmd+mouse overon the func / class / struct / var keywords. You should get a similar behaviour.