I've noticed a number of people on these forums and other sites are reporting iOS 10.3 does not appear as an OTA update on the iPhone 5 - I haven't seen any similar reports from users on iPhone 5s or newer. I also had the same experience with my iPad 4th gen, where 10.3 did not show as an available update in Settings but I can see it in iTunes when I connect my iPad. I'm wondering if because iPhone 5 and iPad 4th gen are the last 32-bit devices supported by iOS 10, could this be related? Perhaps iOS 10.3 wants iTunes to hold it's hand while it updates the file system on 32-bit devices? But I've heard murmurings that "tests" show APFS is not present on the iPhone 5 after the update to 10.3. Is it possible APFS is only supported by 64-bit devices?