Just in case anyone is still interested, it is possible to build SwiftPM libraries and executables for iOS (yes, even executables).
swift build --arch arm64 -Xswiftc -sdk -Xswiftc /Applications/Xcode.app/Contents/Developer/Platforms/iPhoneOS.platform/Developer/SDKs/iPhoneOS15.4.sdk -Xswiftc -target -Xswiftc arm64-apple-ios14.0
It does have some quirks, for example it outputs the built products to the .build/arm64-apple-macosx directory (meaning that swift package clean is required when switching from building for macOS to building for iOS and vice versa. But the built executable does successfully run on iOS which is all that really matters.
I am using this capability in swift-bundler, my tool for creating macOS apps without Xcode (and soon iOS apps too). The ios branch of swift-bundler can successfully build, bundle, sign and run an executable Swift package product as an iOS app on a physical iPhone.