Elif Batuman wrote a piece in the New Yorker called "The Memory Kitchen" about this well known restaurant in Turkey that tries to revive traditional and regional dishes. One of the thing the guy talks about is the death of regional gazozes. Gazoz is a category that covers most sodas that aren't colas--Sprite is a gazoz, 7up is a gazoz.

“Look at what happened to gazoz,” he added, alluding to an indigenous carbonated soft drink. Musa’s article on the subject, “Don’t Touch My Gazoz!,” is an elegy to the golden age of the republic, when every city in Turkey, intoxicated by the introduction of carbonated beverages from Europe, hastened to invent its own local soft drink. “Every region gave rise to its own gazoz, known by its own name,” he writes. “Each region’s gazoz was unique, and was made with the water from its own soil.” Izmir’s gazoz, Cincibir (pronounced “gingibeer”), was flavored with ginger, Niğde’s with raspberry. The glass bottles came in different colors and shapes, and were objects of beauty. Musa describes the “terror and curiosity” with which he and his childhood friends experienced their first gazoz-flavored burps, and the pleasure, as an adult, of arriving in a new city, visiting its tea garden, and sampling the gazoz, whose flavor became inseparable from the memory of the place itself. Today, the Turkish gazoz market, much like the American cola market, is dominated by two corporate brands: Uludağ (once a regional gazoz maker, and now a national soft-drink giant) and Çamlica (part of the Ülker snack-food empire). Both have an amorphously fruitlike flavor reminiscent of Juicy Fruit gum.

Musa paused outside a fish shop, where piles of silver anchovies and reddish-gold mullet lay in bins of ice. Noticing a folding table in the back, he suggested that we go in for lunch. As the owner fried an enormous portion of fish, I contemplated the problem of gazoz. Some of the old brands are, in fact, being resurrected, but of course the golden age described in Musa’s essay is fundamentally irrecoverable. It’s difficult to see any future for gazoz other than corporate distribution or, perhaps, elevation to a niche commodity sold by specialty stores. And it would be hard to imagine Musa viewing with delight the appearance, in expensive tea gardens, of dégustateurs trained to advise diners on their selection of gazoz varietals. But, out of a similar impasse, Musa created Çiya: an idyll that owes its existence to his seemingly impossible expectations.

I'll say two quick things: first, Çiya, Musa's restaurant (which is amazing and a must for anyone in Istanbul) doesn't even serve Uludağ or Çamlıca, they only have Sprite. Second, there are still many regional varieties available. It's definitely the third most popular, but you can find the Niğde gazoz mentioned without much difficulty in Istanbul shops. In a Central Anatolian city not far from Niğde where I have done some research, Niğde gazoz still seems to be the most popular. There was also a cafe in Istanbul--since closed--that served about a dozen brands of gazoz (but that was its whole shtick) so there's still many brands active in Turkey. In fact, just recently a new gazoz (Beyoğlu gazoz, named after a neighborhood in Istanbul) has hit shelves and comes in three flavors: classic, ginger, and lemon (so nothing really too ground breaking).

In America, the most popular soda of the 1920's, or at least one of them, was Moxie. While many soft drinks were originally marketed as patent medicine or nerve tonic, Moxie's problem was that because of gentian root, it actually has a distinct medicinal flavor. I think the flavor has weakened in the past decade, but I'm not sure. The drink, once a national sensation (the term moxie, as in, "you've got moxie, kid," apparently comes from the drink, not the other way around), now only has limited, if not totally regional, distribution. But still exists.

There were a variety of drinks that were mixed at soda fountains (rather than just being syrup plus carbonated water) that have waned in popularity. An egg cream is an iconic New York example, somewhat associated with Jewish culture. It contains neither eggs nor cream, but is instead a mix of milk, chocolate syrup, and seltzer. It's still on the menu of some traditional New York diners, but isn't particular popular even in the city.

Similar drinks, with additional ingredients beyond a typical soda's proprietary syrup and carbonated water, were popular at fountains. Cherry Coke (officially Coca Cola Cherry) was introduced in 1985, but it was an age old soda fountain tradition to add other flavored syrup's to Coke's (or whoever's) syrup, cherry Coke being one of the more popular but not the only example.