Good thread!
Even before I lost my left leg above knee in a road accident 21 years back I preferred leg presses to squats, to keep my arse trim to achieve the big shoulders and chest, small waist and arse, big quads classic x-shape.
Since losing my leg I've twice slightly split my thick polyprop prosthetic stump socket trying to do heavy squats, so I now avoid them completely! Also ATG or parallel squats are bad for the knees, and I need to nurse my remaining one which takes a pounding from everyday use and workouts, or I'd become a total cripple!
Gory detail: when they amputate above knee, they join the free ends of the quad and the biceps femoris together and "hammock" them under the end of the remaining femur, which they grind smooth. It can still hurt like fuck if the bone gets pressed hard into the hammock though.
Though you can still tense these joined muscles to a degree, they're not used in for any normal activity and gradually shrink to a densest state. There's no point in trying to keep them big, as a stump socket is tighter and more stable when they're minimal.
I always had big strong legs from commercial diving. I exercise my remaining leg fully but carefully - no shock loads on my knee or ankle. I do heavy slow leg presses and can usually more than match with one leg what many guys manage with two. Sets of 8 full ROM reps with 9 x 20 kg plates on the machine last time IIRC.