1) A couple layers of duct tape on the back of the hinge leaf.
2) A layer or two of paperboard or non-corrugated cardboard scavenged from discarded packaging.
3) Simpson tie plates, trimmed to fit.
4) Apply spots of a thick spackle or wood filler, then install the hinge leaf, compressing the filler to achieve an exact fit. To prevent the hing from adhering to the spackle, a) wax the back of the hinge leaf with a candle, b) use a layer of kitchen-type wax paper, c) wrap the leaf with kitchen-type plastic wrap. When dry, remove the the hinge leaf and fill any voids with additional spackle.
Note 1: It's odd to me that there is no word for the opposite of deepening: https://english.stackexchange.com/questions/11047/antonym-of-deepen
Note 2: 'Frenchman' is the term for a piece of wood shaped to fill in a mortise or similar recess. For example, when converting a doorway to a passageway, one fills the hinge mortises and striker plate mortise on the jambs with frenchmen.