I think this is one of those "it depends" type of issue. For me personally coffee doesn't affect my singing. But then again, I only have coffee when I wake up in the morning and maybe another cup around noon. The rest of the day I'm drinking water so any effects from the coffee being a diuretic may be countered by the water intake. I don't drink coffee when I sing or right before. But then again, I do drink alcohol at times when I sing. Again, it depends on where I am and what I'm singing. But I supplement with water.

Now, if you're drinking coffee all day and during performances that may be a different story. I should mention that my "cups" of coffee are 8 oz cups. Not the new standard of 16!
I also think that some of these so called issues may depend on what type of music one is singing. Someone singing classical or opera may want a more clear sound and everything counts. Someone singing more bluesy songs may actually benefit from a bit of alcohol or anything adding a certain amount of character to the voice
(without causing damage of course). It may all be situational and personal.

Consuming anything that may dry out the cords just prior to or during a performance may not be a good idea but imo as long as you are countering with hydration after, then I see no problems a few hours before actual singing. At least not for me. How many songs and what type you will sing makes a difference also.