The BBC did wipe some of the early B&W episodes, but they were found in more recent years elsewhere.
The 1970 episodes from series 5 and 6 were originally in colour and are lost or wiped (except for two). The black & white ones on video tape are copies found in the 1990s, which is why they are poor quality.
It's hard to believe that the BBC could do this in 1970 when Steptoe And Son must have already been acknowledged as being such a great show. It would be easier to understand it in the fifties, or sixties. There's no excuse for this vandalism. It's absolutely criminal. Last night I watched the one where Harold goes to a Psychiatrist. I reckon it must have made in the early seventies.