Old three-tab roof with spongy decking Powell, TN.
A lot of factors can cause spongy decking. Usually, the reason is that the rafters may be too broad, around 24-inch wide rafter spacing, and the deck boards might be too thin, like less thin than 1/2 inch.
The fact that poor roof ventilation can cause boards to delaminate or the glue in plywood or OSB to separate and go wrong will be a reason for these boards to be spongy.
This would mean a complete reroof of this project, new boards, and new shingles.