Ok so the scapular plane is about 30 degrees forward (give or take) of the frontal plane. Doing lateral raises in this plane is recommended for safety reasons.
The slight forward lean is a separate recommendation with the idea of putting the lateral head more directly in line with resistance. You can stand up straight and do them, but you hit more front delt than you would if you had leaned over a bit. That's the (possibly over-) simplified version of it.
Upright row is a bit different because the humeral internal rotation already puts the lateral head in ideal position when standing straight up. Even this has some caveats.. e.g. you may find that a slight forward lean is even preferable here because feels better on the shoulder. Or not.. it's highly individual.