A blog post from the National Library of Wales explores how sensitive military and industrial sites were omitted from the published versions of Ordnance Survey maps.
The removal of military installations from OS maps was at its height in the 19th century and the World Wars, but throughout the Cold War and beyond, many sensitive sites were left off the maps entirely. It took the public availability of high-resolution satellite imagery at the turn of the 21st century to render this type of censorship largely ineffective, although labels are still omitted in some cases.
The Ordnance Survey did survey and map sensitive sites, but those maps were military-only. The differences between these military maps and the public maps make for a number of interesting comparisons: see the post for examples.