@JosefPro as I say in my other comment, it's not SwiftData! This is a general problem with FileDocument apps on iOS18 simulators.

iOS 12.5.5 can be installed because it's the highest version for a range of hardware that was dropped from iOS 13 but is still around, eg: iPhone 6, iPod Touch 6. Similar frustration - I just encountered a crash report that's an Apple bug with 14.8 and wanted to restore an old phone to it but can't download.