What about submitting Bitcode when uploading to App Store Connect? Would it take advantage of the change and build an optimized binary for iOS 15 while still supporting the old way on pre-iOS 15 and deliver the appropriate binary upon download?
The mach-o format supports a “fat binary” that contains multiple architectures and then the App Store can only deliver one of those architectures to a particular device. Bitcode lets Apple recompile a different architecture slice using this format. However, since this change is unrelated to the architecture it can’t take advantage of this format.