The most frustrating change I’ve found between Xcode 8 and the Xcode 9 betas was the default remapping of the ⌘-click shortcut to open the Code Structure menu instead of the Jump to Definition command.
Thankfully, there is a way to change it back.
Go to Xcode → Preferences → Navigation → Command-click on Code, and select Jumps to Definition.
Hat tip to user8315745 over on Stackoverflow.
Photo credit: Toshiyuki IMAI