The Early Access program for App Preview Messaging has ended
With App Preview Messaging, Android users were able to send messages using their default messaging application to any of their contacts who were also using Android devices. This feature worked even if the other contact hadn't downloaded the same messaging application.
As of Q3 2020, this program is complete, and users can no longer send messages with App Preview Messaging. User settings related to App Preview Messaging will be removed in future updates.