Basic drawing is now available in Day One iOS starting in version 3.4. You can use your finger or Apple Pencil (iPad only). Drawing is a Premium feature. Scribble is available to all users.
Tap to start a new blank entry, then tap the paper clip icon above the keyboard and choose Draw. If Draw is not in the short menu, tap More > Draw.
Tap Done at the top right to save the drawing.
To edit a drawing, tap the drawing in the entry, then tap Edit. This will allow you to add to the drawing, but original edits cannot be edited.
iOS 14 also offers the Scribble feature from iOS. Use Apple Pencil to write directly in the entry body and text is converted to typed text.