There have been times where I was genuinely grateful somebody honked at me because I was zoned at a green light so no I don't think it's rude as long as you're not harassing the person
The people I lay on my horn to are the ones crossing the intersection and don't have enough room to make it all the way across and block me when my light turns green. I'll lay on my horn until you have enough room. Didn't move through the whole light cycle... I can still have my horn going.