Is it true that people confined to wheelchairs because of paralyzed lower limbs have thin, weak bones in their thighs and legs?

True: The strength of your bones depends a lot on the amount of weight bearing the bone does. People who are paralyzed, if they don't do weight bearing activities like standing in a standing frame, can get weak bones.

Usually: Bones are living tissue that respond to weight bearing. If you compare two similar adults, but one doesn't walk, that persons bones will be weaker. Astronauts who come back from the space station have weak bones from the lack of gravity.