Yes, the city speed limit and the no turns on red rule is posted at every entry point to the city. While not enormous, the signs are large. Additionally, I would venture to say that most people anywhere near NYC have at least heard of this rule - it appears to be common knowledge in New York, New Jersey, Conneticut, and most of Mass. and Penn.
And while it may not be appropriate to get mad at the merely ignorant, you would think the average driver, even having missed the signs at the city limits, might notice when EVERY SINGLE PERSON refuses to make a right on red. You can’t get into the middle of Manhattan without having stopped at dozens of red lights. Furthermore, while you may justifiably get annoyed with someone blocking your way in your hometown, when driving on unfamiliar ground, it is beneficial to at least consider that the locals may have a reason for what they’re doing.
Personally, I’m glad that NYC has outlawed turns on red. Car traffic is too dense, pedestrian traffic is so high, lights are so frequent, and too many people become jerks when trying to look for a break in traffic - the city is safer without it. Even driving around in the wee hours of the morning, there is rarely a opportunity that would be safe to make a turn on red, so it’s not like this rule wastes any of my time.