TfL have very kindly provided data showing how many journeys make use of each OSI in the system. The data was recorded over a four week period in June/July 2018. Unsurprisingly the most popular ones are interchanges between National Rail terminals and Underground stations. There are some surprises, including the accessible route OSI at Woodford which wasn’t used at all. I’m also curious as to why so many people still walk between Hackney Downs and Central when there is a footbridge within the paid areas linking the two stations.
I’ve received clarification from TfL that they have indeed added a new OSI between the above two stations. This means that you can now avoid Notting Hill Gate when changing between Circle/District Lines and the Central Line.