APFS on SSDs Mandatory in macOS High Sierra

Apple Support:

Apple File System (APFS), is the default file system in macOS High Sierra for Mac computers with all flash storage. APFS features strong encryption, space sharing, snapshots, fast directory sizing, and improved file system fundamentals.

When you upgrade to macOS High Sierra, systems with all flash storage configurations are converted automatically. Systems with hard disk drives (HDD) and Fusion drives won’t be converted to APFS. You can’t opt-out of the transition to APFS.

Please make sure to create a good backup (or three!) before upgrading to High Sierra, because Shit Happens when you don’t have one (or three!).