It says  here:

" You can now sort notes by title (alphabetically), or by date modified and date created. You can even organize your notes by dragging and dropping them into a particular order. To sort your notes, simply go to Settings and tap “Sort By.”

Please note: all sort settings will be saved and synced across devices, except for custom sorting. Custom sorting will be device specific."

However, I am unable to 'custom sort' in either Notebook for Mac or on the Web.