A partial specification for APFS was published by Apple in September 2018 to allow the read-only access to APFS drive on even unencrypted and non-Fusion storage devices Even when formatting external drives, it is a better option to go for Mac OS Extended than APFS.I have an internal SSD that upgraded to 10.13 and continued to the current 10.13.6. Many others claim that when you install 10.13 on an SSD, you will be converted to APFS whether you want it or not.