I'm also having issues with this -- I want to be able to target macOS 10.13 as the minimum version for builds, which is a problem when the SDK is nowhere to be found. Some more detail of what I've tried in my quest to locate the missing SDKs. I've tried using xcode-select on the other XCode versions installed (11.0, 11.1) and `xcodebuild -showsdks` to see if they show something other than just 10.15. I also looked for a /Developer folder to see if there were SDKs there, but the folder doesn't exist.
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