The problem still persists. What helped was downgrading Xcode to version 15. This isn't a perfect solution because on macOS 15.1 you can't run Xcode 15 anymore. When will Apple finally fix this? And this is not Today Widgets. Both "New Widgets" and "Lock Screen Widgets" disappeared.

The problem still persists, and forcing the use of Xcode 16 in mid-April before providing a solution to the aforementioned problem is poor.