We've also faced this issue. Trying to get attention from Apple Engineers

NO. We use Xcode 12.5.1 to build enterprise apps and they run fine on iOS 15. Make sure that your build machine is running at least macOS Big Sur (11.6) though

We're having the same issue. Were you able to find a solution for this?

I've noticed a lot of swift-frontend process crashes and Xcode failing to do basic operation, like search for callees. Not sure if it's related, but it's annoying for sure