Of course there was the unmitigated disaster of the BBC junking many
of its archives including whole series of Doctor Who, to say nothing
of numerous comedy series such as Dads's Army. And their radio
archives were similarly junked, especially anything to do with the
genre of 'folk.'

All of this was in the 1970s/80s/90s.

The Beeb even produced radio programmes about its lost archives.

The scandal is that many 'lost' programmes were home-taped by
enthusiasts. These are regularly found and offered back to the Beeb.
Most returns are declined unless they are commercially valuable such
as Clitheroe Kid, Navy Lark, etc.

So the programmes then start to appear on various torrent and usenet
sites. But then the uploaders are served with 'take down' notices -
even if the Beeb has refused acceptance of 'lost' recordings it
doesn't itself have.

These websites explain more ...