At present, this must be done individually for every app, https://www.imore.com/how-stop-siri-learning-how-you-use-app.... When you later install new apps after setting up the device, you have to remember to go into Settings and opt-out again, for every app, forever.
How many people know that iOS devices will default to Siri reading plaintext for all apps, including E2EE messengers?