You’re referring to a z-wave device firmware update feature (the ability to do zigbee device firmware updates was added more recently).
Since the C7’s z-wave chipset is newer than the hardware revisions that came before it, they have indicated in the past they will no longer add z-wave specific features to pre-C7 hubs:
https://community.hubitat.com/t/older-z-wave-stack-vs-c7s-new-stack/53897/11
