Does High Sierra's firmware update preserve legacy osx booting capabilities?

Solution 1:

The firmware update included with High Sierra include drivers to allow the firmware to access ExFAT and APFS volumes. The existing drivers for FAT32 and HFS are preserved. When I upgraded my 2011 iMac to High Sierra, the partition where High Sierra was installed was not automatically converted to APFS since the partition was on an internal HDD. (Conversion only occurs automatically if installing to a SSD.) Therefore, the Mac is still booting macOS 10.13.6 from a JHFS+ formatted partition. So I can confirm the ability to boot from legacy HFS partitions is preserved after installing High Sierra.