Does the limit apply to both Catalyst OS (CatOS) Software-based and Cisco IOS® Software-based Catalyst 6500/6000s?

SÌ. The limitation is due to the hardware design of the PFC. PFC1 or PFC3 support 256 well-known HSRP MAC-addresses. PFC2 supports 16 well-known HSRP MAC-addresses. Therefore, it does not depend on the system software being used.