What is ~/Library/Developer/Xcode/UserData/Previews? (SwiftUI Previews data)
Quick answer
~/Library/Developer/Xcode/UserData/Previews holds the simulator devices that SwiftUI Previews run in (in a Simulator Devices folder). It grows if you use Previews a lot. It's safe to clear with Xcode closed; Previews set up fresh devices next time.
Check its size
du -sh ~/Library/Developer/Xcode/UserData/PreviewsOn our test Mac it contained a Simulator Devices folder and was empty, because Previews weren't in heavy use.
Can I delete it?
Quit Xcode, then:
rm -rf ~/Library/Developer/Xcode/UserData/Previews/*The first preview afterwards takes longer while Xcode prepares a device. Other files in UserData (like your key bindings and themes) are separate; leave them alone.
Questions
Will deleting it remove my Xcode settings?
No, if you only clear the Previews folder. Key bindings, themes and other preferences live elsewhere in UserData.
Why are Previews slow after clearing it?
Xcode has to create the preview device again the first time.