Good question. I looked in 1987 NEC and not seeing this 150% rule. Though I know we were installing 30 amp breakers on nearly all water heaters at that time and was happening long before then as well.Absolutely. We used to be required to have a 25 amp breaker but I am wondering if there was a rule change to allow overcurrent protective device at 150% and allowing next size up rule.
But at same time I wasn't finding anything prohibiting say a 40 or 50 amp circuit either, but not many would run the larger conductor that would be necessary to do so, so 30 amps just becomes the normal since the conductor can be protected by 30 amp device.