They're supported, they're just not upgraded to apfs. At the moment, only SSDs get upgraded automatically. Not entirely clear why, or whether that's a long-term plan.
Ah I stand corrected, cool
> Not entirely clear why
Might have something to do with the fact that APFS was designed explicitly with SSDs in mind.
So an early version of the beta set up Fusion drives with APFS, and then the next version switched them back to HDFS. I don't know the exact issue, but I would roll back like that if I saw there were data corruption/data loss issues.