Macs with Fusion Drives were converted to APFS during the beta testing process in the first macOS High Sierra beta, but support was removed in subsequent betas and not reimplemented.
With the release of the Golden Master version of the software, Apple has confirmed APFS will not be available for Fusion Drives and has provided instructions for converting from APFS back to the standard HFS+ format.
Public Beta testers who had a Mac with a Fusion Drive converted to APFS will need to follow a long list of instructions to convert back to HFS+, including making a Time Machine Backup, creating a bootable installer, and using Disk Utility to reformat their Macs and reinstall macOS High Sierra.
Apple on September 5 published a support document confirming compatibility. When customers with an all-flash machine upgrade to macOS High Sierra install the update next week, their drives will be converted to AFPS. Apple explicitly says “Fusion Drives and hard disk drives aren’t converted.”
