I think the note makes plans and expectations clear to both prom goers and parents. That seems valuable. If what you envision conflicts with their plans, they have time to make other arrangements.
As far as alcohol at parties for kids this.age, it is downrigjt illegal here. If adult hosts chose to furnish it anyway they and their guests would be risking legal consequences. Sneaking it in opposition to clearly expressed house rules seems so ungracious and like such a clear example of immaturity regarding alcohol.
Definitely a greater taboo, as I thought. Interestingly, I'm not sure of the legal position here regarding drinking at home. It is certainly illegal for a shopkeeper to SELL alcohol to somebody under eighteen. But that's on the shop, not the teen or his parents. And in fact most shops are very strict about it - you have to show ID.
Alcohol at parties, though, is such a norm that thinking about it, I think if parents were to forbid alcohol (as of course they are totally entitled to do) the party just wouldn't happen at their house - they'd find another house to go to whose parents were ok with it. I've never come across that happening so far within my son's circle of friends. Although parents often make rules about, for example, where they can and can't go in the house, eg not in the living room, only the family room etc. I quite agree that sneaking alcohol in against an express rule would be ungracious.
I think if there are legal ramifications then it definitely sounds better all round just to specify that there will be no alcohol!