The practice appears to have just been less insisted-on over time. It was finally done away with in 2009 for 'health and safety' reasons (of course it was…)
I checked through a few newspaper reports from that time, this being the most concise (even if it is the Daily Mail) -
Other courtiers and royal visitors are allowed to turn their backs on
her as they exit without any fear of causing any offence.
'It goes back to the times when it was considered terribly impolite to
turn one's back on the sovereign,' he said.
A Buckingham Palace spokesman said: 'There was no major decision taken
about walking backwards.
'The tradition just melted away because in most circumstances it
wasn't realistic in the modern working world.
Apparently, even before then it had been reduced to a mere three steps backwards, then you were allowed to turn.
'The present Queen has always hinted that she's not particularly
fussed by it,' he said.
'I think she takes the view that it's far better for someone to walk
normally than to fall over.'