An easier version of this
White has 20 available moves at the start of the game (from the standard position). After 1.h3 h5, white has 19 available moves. (The h pawn has lost the ability to move two squares, the knight can no longer move to h3, and the rook now has one new move, to h2.)
What is the longest sequence of moves whereby white has at least 1 available move fewer than on the previous turn?