I am not convinced that what this Daniel Lee says makes sense. Demotion isn't really a different mechanic from promotion; it is just another name for the same thing, depending on whether we percieve the origial piece as more or less valuable. Which is a strategic consideration rather than a game rule. If stockfish allows multiple pieces to promote, each according to their own rule and with their own zone, you just make type A promote to B in one zone, and type B to A in the remainder of the board.
The issue could be the "with their own zone" part. When all pieces use the same zone, it amounts to Shogi rules, and Fairy Stockfish does support Shogi.
I am not convinced that what this Daniel Lee says makes sense. Demotion isn't really a different mechanic from promotion; it is just another name for the same thing, depending on whether we percieve the origial piece as more or less valuable. Which is a strategic consideration rather than a game rule. If stockfish allows multiple pieces to promote, each according to their own rule and with their own zone, you just make type A promote to B in one zone, and type B to A in the remainder of the board.
The issue could be the "with their own zone" part. When all pieces use the same zone, it amounts to Shogi rules, and Fairy Stockfish does support Shogi.