I can speak to the quality of bread in the Middle Ages. White bread was the best bread because it was made from unmixed wheat, a “high status” grain. Thus, the eucharist consecrated at mass was to be made ideally from pure wheat; if it was mixed with another grain, wheat had to predominate the mix. Wheat bread was sometimes called paindemaine from “panis dominicus” or “the Lord’s bread,” i.e., Jesus. Peasants grew wheat but could hardly afford to eat it unmixed; it was a cash crop meant for sale or for their lord. Wheat was just one grain from which bread could be made. Rye, barley, oats, and even beans, peas, or bran might also be used in various combinations. These grains, of course, made the bread darker, like “brown bread” and even “black bread,” which signified lower quality and lower status. So, the further down the social scale, the less wheat in bread. Some monasteries would make distinctions between “monk’s bread,” “servants’ bread,” and “almsbread.” At Worcester priory in the 1240s, on major feast days everyone got bumped up to the next level of bread quality.

See Bridget Ann Henisch’s lively discussion of the quality of bread in Fast and Feast: Food in Medieval Society, from which I draw in this answer.

White bread as we know it (Wonder Bread and the like) is a product of industrialization and mass production at the turn of the 20th century. Its uniform shape, softness, and particularly color were marks of quality and cleanliness--other types of bread could include other types of grains or nasty additives like sawdust. (The situation is similar to the one Whoosier describes in the medieval period, but on an industrial scale.) Bread companies marketed their products based on claims of purity, often referring to their facilities in the early 20th century as "sanitary bakeries." (Making one wonder what the other bakeries were like.)

In short, yes, white bread was consistently considered a luxury food item until the 20th century when industrialization made it commonplace. BUT, as your WWI example shows, early in the century it was still kind of a big deal. Furthermore, wartime food shortages were pretty severe in Germany. I can't speak specifically to the bread soldiers received, but civilians often ended up with unpalatable breads made with a mixture of grains along with other... ingredients.

For the development of white bread see, appropriately, Aaron Bobrow-Strain, White Bread: A Social History of the Store-Bought Loaf. For the German case, you can try Belinda Davis, Home Fires Burning: Food, Politics, and Everyday Life in World War I Berlin.