Same with an Ultra 1st Gen, which can't get watchOS 27, paired with a iPhone on iOS 27 Beta 6 (but hasn't worked since Beta 1). Developer mode is on the watch, but doesn't appear in Xcode 26.6 or 27.0 beta 4

Confirmed it's definitely not an issue with Xcode 16.2 (16C5032a), so using that again as a workaround for now. Submitted feedback: FB17262993

Yes, seems to be an issue with the latest Xcode Version 16.3 (16E140). It's no longer possible to build/install to a physical iPhone/iPad, simulators are unaffected. I will often author an app using Swift Playgrounds on the iPad, open the .swiftpm file via Xcode to install directly onto a device for debugging. This is no longer an option.