What is SDKStatCaches.noindex in DerivedData?
Quick answer
~/Library/Developer/Xcode/DerivedData/SDKStatCaches.noindex holds small .sdkstatcache files, one per SDK version (like iphonesimulator27.0-24A430-….sdkstatcache), that Xcode's build uses. It was 3.1 MB on our Mac. Safe to delete, but there's little to gain.
What it is
Each file is named after a platform, SDK version and build: iphoneos27.0-24A430-…, iphonesimulator27.0-24A430-…. Old SDK versions' files stay after you update Xcode.
Can I delete it?
rm -rf ~/Library/Developer/Xcode/DerivedData/SDKStatCaches.noindexXcode recreates what it needs on the next build.
Questions
Why are there files for old SDKs?
Xcode doesn't remove caches for SDK versions you no longer have. They are tiny and safe to delete.