Interesting & informative. Good post.
I didn't know TM's drive must be formatted as HFS+, although I should have realized as my TM drive was formatted before (so, HFS+) I upgraded to High Sierra (and as my MBP 15 drive is SSD, it auto-converted to APFS). Moreover, I had not noticed any changes as to how TM was completing backups after my upgrade to High Sierra. Nonetheless, I'll keep this key detail in mind (I periodically erase / reformat my external, TM drive) until such time TM provides for backups to an APFS formatted drive.
Thanks for the link.
Add to that neither Apple RAID or SoftRAID currently support APFS, but SoftRAID 6 is due out soon and will include APFS support.