mikas
macrumors 6502a
^Apple tells us like this today:
"When you install macOS High Sierra on the Mac volume of a solid-state drive (SSD) or other all-flash storage device, that volume is automatically converted to APFS. Fusion Drives, traditional hard disk drives (HDDs), and non-Mac volumes aren't converted."
and
"Disk Utility in macOS High Sierra can format most storage devices using either file system. If you need to manually reformat, consider these points: "
...dot no 3: APFS is optimized for solid-state drives (SSDs) and other all-flash storage devices.
So I guess it's (at least kind of) supported, just not optimized for other than all-flash or pure-ssd types of devices.
"When you install macOS High Sierra on the Mac volume of a solid-state drive (SSD) or other all-flash storage device, that volume is automatically converted to APFS. Fusion Drives, traditional hard disk drives (HDDs), and non-Mac volumes aren't converted."
and
"Disk Utility in macOS High Sierra can format most storage devices using either file system. If you need to manually reformat, consider these points: "
...dot no 3: APFS is optimized for solid-state drives (SSDs) and other all-flash storage devices.
So I guess it's (at least kind of) supported, just not optimized for other than all-flash or pure-ssd types of devices.