Since macOS High Sierra, all devices with flash storage are automatically converted to APFS.[19] FileVault volumes are also converted. As of macOS Mojave, Fusion Drives and hard disk drives are also upgraded on installation.[20] The primary user interface to upgrade does not present an option to opt out of this conversion, and devices formatted with the High Sierra version of APFS will not be readable in previous versions of macOS.[19] Users can disable APFS conversion by using the installer's startosinstall utility on the command line and passing --converttoapfs NO.[21]
This wouldn't affect many people if they upgraded to newer macos systems, but it could present problems for any (including me) who still sometimes use much older systems. In that case it might prove difficult to move data between newer systems and older ones unless hard drives or networks are used. Knowing that the automatic conversion of flash drives to AFPS can be prevented is perhaps not good enough, as the knowledge may come after one's hand has been bitten.
Actually I'd probably get round it by converting to FAT - but it would be a retrograde step in some situations, and might sometimes give problems. Another way would be to convert to NTFS - assuming that one's system had NTFS capability.
Is this really a problem?