It was introduced on 6 September 2009: https://tfl.gov.uk/info-for/media/press-releases/2009/september/introduction-of-pink-oyster-route-validators-at-selected-

It was introduced on 6 September 2009: https://tfl.gov.uk/info-for/media/press-releases/2009/september/introduction-of-pink-oyster-route-validators-at-selected-stations https://tfl.gov.uk/info-for/media/press-releases/2009/septem... I used this frequently at some point, but it is now so long ago I can't remember where. Especially with many London Overground routes, it's not necessarily slower.