It’s a gray area, in my opinion. I put my feet on the bottom of the bulkhead from time to time to stretch out my legs / readjust positions. Many bulkheads simply do not have enough legroom, especially on Delta where there’s often minimal or no cutout for feet, so I’m sympathetic when people adjust their position to try to get comfortable. I think it’s ridiculous (to say the least) when people put their feet on the bulkhead above the height of the seat cushion, but that’s a matter of personal opinion. At the end of the day, if another passenger’s behavior doesn’t affect me, it’s none of my business.