Some Macs running up-to-date versions of their operating system may not be running the latest EFI firmware version, exposing users to firmware attacks. Experts made this discovery after collecting and analyzing telemetry data from over 73,000 real-world Mac systems. Some Mac models received regular EFI updates, others only after certain vulnerabilities came to light. 16 Mac models listed in a table at the end of this article have never received a firmware update. The 2015 21.5 iMac has the highest occurrence rate of incorrect firmware with 43% of sampled systems running incorrect versions.
Source: https://www.bleepingcomputer.com/news/apple/many-up-to-date-macs-not-getting-efi-firmware-updates/