Yup, you read it right

. I knew Apple was having some issues with Fusion drives and APFS - which is why in early betas fusion drives would be automatically updated to APFS, but that was removed in later betas. In the previous (non-GM) build, you couldn't even convert a fusion drive to APFS after the fact. The only way to get APFS was to wipe your existing fusion drive, recreate and format it as APFS. I did just that, and restored from a Time Machine backup. Everything seemed to be working fine, but there may be a few show stopping bugs that I haven't encountered, forcing Apple to remove the ability to use APFS on fusion drives.
With all that being said, as frustrating and annoying as it is to have to wipe my computer clean and set it backup using HFS+, that is the price you pay for using beta software

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There still seem to be a lot of issues with APFS...which has me really concerned for this release.
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As far as I can tell you can't install the GM on fusion drives formatted as APFS at all - the installer simply won't let you. I've been using APFS on my fusion drive for a few weeks now, it seems to be okay. I haven't noticed any issues that seem file system related (corrupt files, etc.), but there have definitely been some annoying slow down issues, and a bit more than usual spinning beach ball of deaths - it is quite possible those issues are related to APFS and the fusion drive. I'm currently backing up my system and reformatting it back to HFS+ so I can install the GM - we'll see how things run after!