Recently I came across a commercial kitchen serving line that had a 3 phase panel installed within the modular equipment line. The connected loads were various single phase 208v heating appliances. Since the engineer did not coordinate his drawings with the kitchen supplier’s shop drawings, the plans showed the feeders to be single phase 208v. The installer connected his single phase 208v to the three phase sub-panel and arranged the overcurrent devices to A & B phases to accommodate his installed feeders; C phase was unused and blanked off.
A&B phases were balanced and the feeders were properly sized for the load.
Unless I’m missing something this is not a code violation, agreed?

If there is no high leg like you say, then I don't see a code violation. It is odd that the loads are only on the A & B phases, but maybe that is for balancing purposes.

The modular supplier probably supplied a 3 phase panel so it could be fed with either 3 phase or single phase 208. Since the electrician fed it with single phase, you simply have a phase that can't be used. I believe Nick is only commenting that if the feed had been from a 3 phase Delta (120/240) then it would be required to leave the "B" phase unused to indicate the high leg present on site.
It's not uncommon to have single phase panels tapped from 3 phase services and ok by code. I just presents some balancing issues for the 3 phase service (because one phase may not be as loaded as the other 2).

As long as the engineer had the loads accurately accounted for and balanced in the service, I see no problem.