When an area is quite well mapped it needs to be kept up-to-date. A mapper may have visited somewhere to map it when it's not normally somewhere she might visit, so any new developments might not get mapped as it is built. These new developments all get new postcodes, so showing a map with all the recently added postcodes might help mappers find places that need another survey.
I've put together a map that shows all of the postcodes added since the beginning of 2016. You can see it here http://pcdates.raggedred.net/#10/52.7159/-1.3149. There would be quite a lot of markers in some areas so I use the excellent MarkerCluster plugin to make the markers more manageable. Each marker is a single centroid. If you click on it it shows a popup with the postcode and start date in it.
If it is useful please let me know what you think. Would you like anything changed? If people like it I will maintain it as each new release of postcode data id available, which is currently four times each year.