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.
Since 1995, it had been established that 20 is the least number of moves required for the worst-case solution. In 2010, Tom Rokicki showed that no configuration demands more than 20 moves.