C++ interop supports back deployment for all features except foreign reference types. The standard library overlay (which adds things like initializers to convert Swift Strings to C++ strings and vice-versa) is bundled with your app via a static library. Foreign reference types will be marked as unavailable on older OSes and only back deploy to iOS 16.4, macOS 13.3. Here is a link to the relevant documentation: https://www.swift.org/documentation/cxx-interop/status/#minimum-deployment-version-for-mixed-language-application