iPhone Duo AppsThe model and timing matter
Apple's launch announcement says support for Apple Pencil USB-C on iPhone Duo is coming later this year. That is a later feature, not a launch-day promise. The claim applies to the model Apple names. Read the official announcement before buying an accessory for this purpose.
A note-taking app's existing handwriting tools do not establish that every Pencil feature will work on Duo. Check the app publisher's current support statement and the version needed for the feature you want. Our watchlist keeps unverified apps labeled accordingly.
What developers can prepare
PencilKit gives an app a drawing canvas that captures touch or Pencil input. Its drawing objects can be saved with the app's content. That makes drawing, saving and reopening a useful task to review in an existing app.
Check that the canvas remains usable when its available space changes, and that the drawing survives navigation and backgrounding. Those are preparation tasks. Pencil input, device behavior and the final app experience still need testing in the supported environment.
Keep a record of the device, OS, app version and exact Pencil model when testing becomes possible. The compatibility guide explains how we distinguish those results from an announcement.