However, in another update that appears to be server-side, the formatting options are beginning to appear in old notes as well. These options show up when you tap the A icon on the left in the toolbar just above your keyboard.
Source – Google
Regardless of this limitation that keeps this feature working within the Android app only, the update is still a welcome addition for users who rely on Google Keep to organize their notes and ideas. Text formatting can help to make notes more visually appealing and easier to read, not to mention emphasize important information or create different sections within a note.