How can I enable Safari 13.1 (Catalina) to render specific unicode characters?
Display of unicode characters in any app normally requires that a font with that character be installed on the machine.  Black Truck (U+26df) is not in any MacOS font for 10.13, 10.14, or 10.15 as far as I can tell, so you would need to install one of those mentioned on this page to see it.

MS Word comes with one of them,  Segoe UI, so if that app is present and that font is available to other apps, then you should be able to see 26df (⛟).