IMO- the "don't back pedal and your chain won't fall off is a cop out of lazy or inattentive mechanics.

While it's hard to justify backpedaling, sometimes there's a legitimate need to, such as to position a pedal for starting from a stop, or to quickly lift a pedal on a mtn bike to clear a sudden obstacle.

If all is OK the chain shouldn't fall off when back pedaling. Obviously there's no derailleur at the bottom of the chainring to prevent it, but by the same token the chain should stay on the chainring when pedaling (forward) without touching the FD cage, or without the need for an FD at all, such as in a 1x9 setup.

Usually when the chain falls off backpedaling it's because the RD isn't vertical, so the lower loop has more angle (cross-chain) than the upper, and is a sign that the RD hanger isn't square. You can do a quick eyeball check by sighting from the top with the chains directly above each other. The lower loop should be parallel to the upper and able to hide under it nicely.

With a bit of attention any decent mechanic should be able to keep the chain from coming off when backpedaling except, maybe, in the big/big crossed combination.