I can't think of any situation I'd ever be in where it would be required. Can't do it anyway with my split fins.
Like many who have already posted, I back fin frequently during dives. The ability to do it does indeed depend upon your fins, but having split fins is not in itself a barrier.
There are two very different methods for back kicking. One uses wide fin sidewalls for the propelling surface, and the other uses the fin surface for the propelling surface. I learned the latter method first, and I now use the former. If your fins do not have wide sidewalls, you must use the second method.
As for split fins, I have two contrasting experiences to describe. I was once teaching a refresher class for a father and son, and the son showed up with Atomic split fins that were ridiculously too big for him. He insisted they were his, but he had obviously walked away with his last diving experience with someone else's fins. I gave him my fins, which were still too big for him, and I used the ones he brought, even though they were so big the spring straps barely touched my heel. When I needed to back fin during instruction, I used the fin surface technique
and I had no trouble back finning whatsoever.
In the second case, I tried to show someone with a different pair of split fins how to do it, and it was a total failure. She was doing it perfectly, but the fin was so soft it was simply flopping and accomplishing nothing. I have had a similar experience when borrowing floppy blade fins. I could still back fin with them, but it took me at least twice as many kicks to cover the same distance. I therefore conclude that the most important quality of the fin for that technique is stiffness, and you should be able to back kick with a stiff split fin.