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New versions of Xcode leave behind cruft from old versions. The following directories usually will have some files/directories that can be removed after you upgrade to a new Xcode release.
iOS Device Support
On older Xcode versions, check this folder:
~/Library/Application Support/iPhone Simulator/
I hadn’t done a clean-up between Xcode 8.4 and Xcode 10.1. Cleaning up all pre-Xcode 10.1 stuff netted me 93GB. When your drive is only 256GB in the first place, that’s quite a savings.