I guess so many of the games I've made, played, and seen videos of are platformers, so I just sort of took jumping for granted.
The intended control scheme is arrow keys + Z (which is what most platformers I've played on keyboard use), or WASD + J (which IIRC is what the Distorted Travesty series uses, which is the only non-jam platformer I've played that uses WASD). (This says arrows + ZXC has been standard since Cave Story.) I've gotten at least one other comment on a different game about Z to jump; is it really that uncommon or is it just people not used to 2D platformers on PC?
(Oh, and also I'll almost certainly make a post-jam update that adds remappable controls. I've already written code to do that; only reason I didn't include it was because I didn't want to include too much code written before the jam.)