macOS continues to use the HFS+ file system for Time Machine drives for the time being. If you back up your Mac using Time Machine, you won’t be able to use APFS, either. If you have an older Mac, you’ll need to keep using Mac OS Extended or use an alternative like ExFAT instead.
The biggest downside to using APFS is that Macs with older macOS versions (macOS 10.12.6 Sierra and older) can’t read, write, or otherwise access drives that use it.
