Will a 72 x 24 x 1 3/4" Maple tabletop sag if supported by only four legs?
There is a cool website that calculates sag based on a variety of inputs.  https://www.woodbin.com/calcs/sagulator/

It also naturally depends upon where you attach the legs.  If they were set in from the ends a fair bit, that would go a long way in preventing sag.