I'll go against the grain.
If you format it to HFS+, it will be usable with older Macs, if you ever have the need to connect to one.
I see no reason to be forced into APFS, just because Apple thinks we should go there.
The internal drive on my new 2018 Mini boots and runs from HFS+, and will for the life of the machine...
(in a similar vein, I still use POP for email, and will continue to use it until the ISP's make it impossible to use any more...)
Speed test differences on my new T5 drive using encryption and not using it with APFS is 430mb/sec with encryption... or 480mb/sec without encryption. I decided to go with encryption. Starting to copy everything over now (files etc). Mainly using this for video editing.
Yes, that is still the case due how Time Machine catalogs data at a file system level that APFS doesn't support. Time machine will be updated in time however it will format and external drive as required.
I have my backup Samsung T5 formatted in Mac OS Extended Journaled. I used Carbon Copy Cloner to clone the original Fusion Drive on my iMac. Since then, I've upgraded my iMac (late '12) to Mojave and it reformatted the Fusion to APFS without issue. Some people say that isn't possible, but it works great for me. I should probably go ahead an update my Samsung T5 so it has APFS as well. It starts up great on it anyway.....small, light, speedy....great value that T5.
Update: Pleased with the T5 so far. Formatted it as APFS and copied 250GB of photos from the Mac to it which took about 9 > 11mins. Then the same photo backup from Mac to usb 3 external hdd which took over an hour. Quite a difference