I don't know about iPhones, but on Android it probably depends on keyboard installed/configured itself, until developer forces its own behavior. I've seen only

I don't know about iPhones, but on Android it probably depends on keyboard installed/configured itself, until developer forces its own behavior. I've seen only single app that misbehaved and was sending the messages on return key - Xabber. Others just let me insert newlines as I'd expect.