Back in May 2017, Apple filed a patent application for a way to communicate with Siri by writing questions and responses using iMessage. Siri things with the keyboard directly in the Siri interface. This works on iOS 11 and iOS 12, and I'm sorry, this is working with any iOS running iOS 11 or higher, but there's one downfall to this: You can only do one or the other via the Home button or Side button shortcut – type or talk. "Hey, Siri" on supported devices to have verbal conversations either way.
To switch from voice to text only, just go to Settings -> General -> Accessibility -> Siri, then toggle on "Type to Siri." That's it. "Siri" in the Accessibility options, then go to "Siri & Search" in the Settings app and make sure "Listen for 'Hey Siri'" or "Press Home for Siri" / "Press Side Button for Siri "is turned on.
Voice Feedback is set to" Always On, "though so there will be written responses and data cards.
Siri something via typing, just long-press your Home or Side button, depending on what iPhone model you have, and the Type to Siri interface wants to show up with the keyboard.
You can keep the conversation going by. Apple one.
Siri interrupting music playback feature is the only thing that's missing from this "Text to Siri" feature. Do not Miss: 100+ Cool New iOS 12 Features You Did not Know About