All I know is that the Time Machine drive should be formatted the same way as before: HFS+ (Journaled).
I'm assuming you are planning on using a current TM drive. When you start using HS, you should not need to do anything to the current TM drive. As a matter of fact, if you re-format a TM drive, you will first destroy the current backups. Secondly, when you try to get TM to use that APFS drive, it will tell you it can't and ask you to re-format it back to AFS+.
Bottom line; don't do anything to the TM drive. Well, it might be good to check the date of manufacture and change your bets on how much longer it will last...