In recent macOS release cycles, Apple has followed a fairly regular pattern. The last minor update to the previous major release of macOS, such as 10.13.6, has brought EFI firmware updates across all models, in preparation for the first release of the next major version. That hasn’t worked out with Catalina: if you have installed its first release, your Mac will have undergone firmware update, but Macs still running Mojave and High Sierra have, for the moment at least, been left running the previous firmware.
In the normal course of updates, we should expect Apple to release security updates for High Sierra and Mojave very soon, which will install the same firmware updates as came with Catalina. Although that may well still happen, there’s something else rather odd with these Catalina-only firmware updates: they’re mostly older than the last Supplemental Update (the third) for Mojave 10.14.6, and its parallel security update for High Sierra.
It’s not as if the Catalina firmware isn’t generally compatible with the last releases of Mojave. Many of us who’ve been beta-testing Catalina also dual-boot those same Macs from Mojave. Although a few have reported kernel panics which might result from incompatibilities between the Catalina firmware and Mojave, those don’t appear widespread. Indeed, those kernel panics seem if anything less common than those still reported in Mojave.
The most likely reason for this disparity in firmware versions is that Apple’s engineers have been focussing on shipping Catalina first and foremost. Now that has been released, they can bring High Sierra and Mojave kernels (in particular) up to full compatibility with the new set of firmware, and release those in a forthcoming security update.
If that’s the case, then one question remains open: in the normal course, Sierra security update 2019-005, released on 26 September, would be the last update to Sierra as support is withdrawn following the release of Catalina. Had these firmware updates shipped with Mojave 10.14.6, as had probably been intended, then Macs still running Sierra could have installed them. If Apple doesn’t release any further security update for Sierra, then it ends its support cycle in arrears of High Sierra and later. That means that it will already have known bugs and security vulnerabilities.
If you were planning to stay with Sierra for the time being, you’ll be watching closely for the next security update, bringing previous versions of macOS up to parity with Catalina’s new firmware. If Sierra isn’t included in that, you may wish to reconsider your decision and perhaps upgrade to High Sierra or later, which will at least continue to be supported by Apple.
It’s hard to envisage Apple leaving High Sierra and Mojave with older firmware, but I wonder how long their next security updates will take.