An old post covering changing attitudes to archiving TV shows, with the prime example of Doctor Who as a show missing episodes that were destroyed by the BBC and so far never recovered.
Wipe Out: When the BBC Kept Erasing Its Own History
In the 1950s, '60s, and '70s, the BBC was so intent on erasing its original programming that employees could wipe a show with just three words on a generic form: "no further interest."
mentalfloss.com