Dairy - milk, cheese, yoghurt; and eggs, and rice are all catalysts for the body to make mucus (in the rice it is the Gluten).
Yoghurts and the like, though the yoghurt is another culture, still leave a coating on the throat, vocal chords and esophagus.
If you want to test this, cut out all the milk, cheese, yoghurt, eggs, rice from your diet for a week and see how much less mucus is in your sinuses (masque), and how much less 'coated' or furry are the throat, vocal chords and esophagus.
Sugar may not only leave a sticky coating, but often has us reacting by producing more sputum, and you don't want to have to fight drinking excess saliva while trying to sing.
The alcohol - is just a rationale for those inclined to alcohol. There are sugars there, too.
To relax the throat and keep the vocal chords (which are muscles) warm, a bit of warm / hot tea (tannic acid and caffeine) - with a bit of lemon juice doing no harm, or a chamomile tea if one needs to take the edge off the adrenaline nerves so many of us experience before performing.
You don't have to 'abandon' the foods or the food group entirely, but I would not at all recommend consuming any dairy, at the least, on a day you are going to perform.
P.s. With a cultural dietary habit as huge as the Nederlandish one centered around diary products, -- milk, cheese, etc. it might be rather difficult to 're-train' yourself to another habit of foods consumption. Elsewhere, even high school choral directors routinely advise their singers to avoid these foods the night before / day of a performance.