Apple - Does Time Machine support APFS?

Time Machine supports APFS as a source and not a destination. You can't backup to an APFS destination disk and Time Machine will inform you that the disk needs to be HFS+ if you attempt to do this.

Right now, there's nothing you need to or should do. Your internal disk has been converted to APFS and your backup disk is still on HFS+, so Time Machine will function as it always has, and will continue to inherit previous backups despite the difference in format. On a future date when Time Machine supports APFS destinations, you can convert your backup disk.

Until Time Machine is updated, it does not support APFS on the backup disks.

Apple needs to resolve the linkage to folders first. Hard links to directories are not supported in Time Machine, Apple will need to reprogram TM. I'm sure when they have Time Machine updated, they will make the change to APFS seamless.

In my opinion, you should avoid APFS on external hard drives, as you will have trouble restoring it if the disk has serious trouble. See my post here: https://discussions.apple.com/message/32385656#32385656