I assume no one in Europe knows what Highball is. I also thought that it is a Japanese English but unfortunately not.
Highball is a mixed alcoholic drink based with spirit and non-alcoholic mixer. The well known samples are Cuba Libre and Scotch & Soda. In Japan, whiskey with soda is called Highball and it is extremely popular. All izakaya style pub restaurants offer Highball and also you can by it in supermarkets and in convenience stores. By the way, most of the Japanese will not understand Cuba Libre. Then, it is normal to mix Scotch with still water but not with soda.
Then, there are flavored Highball in Japan which is called Ginger Highball and Cola/Coke Highball. These two are made from ginger ale or cola instead of soda. Highball should be based with whiskey but cheaper version exists and other spirits are used instead of whiskey.
It is interesting but whiskey is drunk in Japan but almost no izakaya pub restaurant offers whiskey alone. Not only whiskey, gin or vodka cannot be ordered as a single drink. The restaurants offer mixed alcoholic drinks as GIn Tonic or Screw driver but a shot of vodka is never listed on the menu.
Highball can be ordered on Shinkansen too. Then, sometimes offer with a snack for a better price.