As pointed out in this question, the Apollo command module had crushable ribs along one edge of the base of the cone, and crushable struts that suspended the crew couches inside the vehicle. I've added some diagrams from CSM06 Command Module Overview pp 39-52.pdf below.
By my count, 19 command modules returned to Earth: AS-201, AS-202, Apollo 4, Apollo 6-17, Skylab 2-4, and ASTP. Most (all?) are in museums. Do any of these surviving command modules show visible deformation of the impact attenuation system (either internal or external) ? Photographs would be helpful.