As long as we’re talking soda, I miss locally bottled pop sweetened with actual cane sugar. Where I grew up there were three local bottlers into the late 70’s. Is the “Ma’s” brand still out there anywhere in any form?
Not only that, but Quaker’s been doing that for at least 15 years now --I worked at Quaker in the late 90s, and Quisp was in limited distribution even then.
At that time, Quaker also had a line of bagged cereals, and there was a product in that line that was the exact same cereal as Quisp (but had a different name), and had a lot better distribution. Alas, the Quaker Bagged line is now extinct, and you can’t get Quisp that way, either.
Correct me if I am wrong, but as best I can remember, Quisp is the same cereal as Cap’n Crunch save for the shape of the pieces and the picture on the box. Ditto for Quake.
Golden Nuggets breakfast cereal in the UK. A bit like Cap’n Crunch but tastier - less sweet and with more of a ‘grain’ taste. It was the biggest treat of my childhood, only for special occasions and holidays.
They reintroduced them in the 1990s but they’d changed the recipe. 
I want my Golden Nuggets!
There’s a Facebook group called “1980s Crisp Packets”. You wouldn’t believe some of the bizarre potato chips you used to be able to get in the UK: chocolate (not chocolate coated but some ersatz ‘chocolate’ powder that tasted nothing like it), sausage and tomato, hedgehog flavor.
Nabisco Swiss Cheese crackers.
Apple Newtons.
Who else remembers Team Flakes? Even as a kid, I preferred those over other cereals.
In fact, now that I think about it, there were about a million cereals that were just Cap’n Crunch dough forced through a different extruder nozzle. Kaboom. Now there was a unique cereal. Uniquely bad, but nonetheless. Boo Berry, which may even still be around, was vile beyond what words can express, but at least it wasn’t yet another Cap’n Crunch shape.
I have three or four (small) boxes of Quisp on top of my fridge. I got them at Target a few months ago, where they were on closeout for $1.24/box. I believe they still sell them at the big Target (Super Target?) but mine isn’t the Super variety–I just got lucky that the spouse spotted them on the shelves.
I believe you can order them on the net as well. Why yes, yes you can! 
I miss Jello Pudding Pops.
Entenmann’s Cream-Filled, Fudge-Iced Devil’s Food cupcakes.
I miss those tremendously.
I also used to like Pepsi Twist (the Pepsi with a little lemon flavor).
A cereal that’s still around, but which has been reformulated into something only barely resembling the original : Cracklin’ Oat Bran. Circa 1980, it was wonderful. My mom would buy it in lieu of cookies, and apparently it was nutritionally equivalent. From what I recall, it was redesigned after a big stink about its high fat and sugar content. Tastes like crunchy cardboard now.
Bar None candy bars
Frusen Glädjé ice cream
Dannon frozen yogurt pops (raspberry yogurt, dark chocolate)
Breyer’s ice cream - before they put that additive in it to make it more “creamy” which makes me think of jellied snot. I used to love the Breyer’s vanilla fudge swirl, the two flavors were slightly different consistencies and I would sort of pull them apart with my spoon and eat the vanilla and cocoa-y fudge separately.
Not discontinued, but I miss Dinty Moore Beef Stew. It was a camping staple when I was a kid and we camped as a family. I would occasionally grab a can as an adult to wax nostalgic with a ton of Club Crackers mixed in. I don’t eat factory farmed animal products any more, so it’s off the table, but I still miss it and the old ice cream.
Oh, man, I liked the diet Pepsi Twist (IIRC it came in either lemon or lime). I would buy that by the twelve pack even now (I don’t drink much soda anymore) if I could get it.
That reminds me, Entenmann’s Apple Strudel. Yeah, it was hyper-sweet but that’s what I liked about it. The best part was the unfilled “handle” on the one side that I liked to pull apart in layers, saving the sugared top for last. I looked for this for several years before I learned it had been discontinued. 
Years ago, Carnation made a hot cocoa mix called Chocolate Meltdown. It was sooooo chocolaty and thick. Unlike their current line which is thin and taste nothing like chocolate.
Heinz Hot Ketchup.
Not Heinz Not & Spicy Ketchup, which is made with Tabasco sauce…this is the old stuff they stopped making 15 or 20 years ago. It was somewhere between ketchup and cocktail sauce in terms of spiciness, as I recall.
And this is not a ‘lost food’, because it is obtainable, but not easily…Heinz Mustard. Every other restaurant in America has it on the table, but you simply *cannot *buy it in the grocery store.
Go to a market in a Mexican neighborhood. They will have Pepsi, Coke and any number of other flavors in glass bottles made with real sugar.
Wheat Nuts. They were a snack food back in the 70’s, made in the shape of nuts. Artificial nuts. They didn’t really TASTE like nuts but… ohmyfuckinggodwheretheyGOOD!!! All these years later I can still taste them…
Seconded. This is what I came in to say. Breyer’s Vanilla was one of the best ice creams ever – cream, vanilla (not vanillin) and sugar. Zen-like simplicity, with no emulsifiers, gums, or additives. Nowadays it’s hard to find something to take its place. Breyer’s was national – I could get it in Utah or California as easily as in New York, New Jersey, or Massachusetts. Any other simp;ly ice creams I’ve come across have been local and often short-lived.
I also miss Shake-a-Puddin’:
http://www.boston.com/business/gallery/retro_foods?pg=10
It required mno refrigeration and no milk. All you needed was water and five minutes. It was the perfect transportable dessert, which made it ideal for camping and hikes. My scoutmaster hated the very idea of it, but it’s no different from all those dehydrated foods you can get it camping stores anymore.