To pile on, there are different stay types and different configurations. Removable metal (aluminum) or carbon fiber stays sometimes are just there to lend shape and stiffness to the back panel. More effective designs also use the stays to mechanically connect the hip belt and shoulder yoke and therefore are critical to overall suspension dynamics. Take it/them out and you have a fancy flour sack.

More sophisticated designs will pair the stay(s) both with a thermoplastic frame sheet and a perimeter frame hoop of aluminum, spring steel, delrin or cf. If a pack uses hoop to connect the yoke, hip wings and load-control straps, the stay can often be removed for lighter loads with no performance penalty.

Shorter version: it depends.

Cheers,