OP, are you referring to the "Hey Siri" feature... or the fact that when you activate siri, you can see each word transcribed AS you say it? It sounds like your speaking of the latter, but all replies seem to discuss the former.
If the latter, this is a great feature which i'm sure will make its way to the speech-to-text button on the keyboard.
If the former, please ignore everything i've said 😀.