You may not have all the facts, or you may be confused by the facts.
Macs running Mohave or later work just fine with a hard drive formatted in APFS for Time Machine.
Actually, if you start with a blank drive....which you should.....the Mac will automatically format the drive in APFS (Case Sensitive) before the first Time Machine backup begins.
The important thing to note here is that the hard drive has to be connected directly to the Mac.
Now, suppose that you want to connect a hard to a network location.....like an AirPort Extreme or another router that supports The Machine.
That type of setup requires that the hard drive be formatted in Mac OS Extended (Journaled), also known as HFS+.
A drive formatted in APFS won't be recognized at all in this type of setup.
If you have a Time Capsule......things get really complicated. The Time Capsule hard drive can only be formatted in Mac OS Extended (Journaled)......But....the sparsebundle file on the Time Capsule hard drive that contains the backups is actually formatted in APFS (Case Sensitive). Weird.
Gosh, I wonder what could go wrong here with the extra complexity. But, maybe more than you really were asking about.