While it had been known since 1995 that 20 was a lower bound on the number of moves for the solution in the worst case, Tom Rokicki proved in 2010 that no configuration requires more than 20 moves.
From 1995 onwards, it was known that 20 represents a minimum on the number of moves in the worst case. By 2010, Tom Rokicki proved that no configuration requires more than 20 moves.