Not sure if this has been done before…anyway, my nominees are Kellogg’s Pop Tarts and Kraft Deluxe Mac and Cheese. Still OK, but not near as good as they used to be.
Anything of American made chocolate, American chocolate these days seems to be made mostly of paraffin, food coloring, and skatol.
PopTarts were the first thing I thought of. There was a distinct “background taste” (?) that I can’t even describe but is definitely no longer there. It was only present in the frosted varieties.
Ditto on the Poptarts.
Chief Boyardee seems to have gotten sweeter than I remember.
Twinkies. I haven’t had a decent Twinkie in decades. It’s like they purposefully started making them with rancid oil.
Blech!
Also apples, strawberries, and tomatoes. They all seem to have been bred out of any recognizable flavor or texture. The ones from home gardens still taste right though.
And sweet potatoes seem to taste better than they did. They have a lot more natural sweetness. I’m not sure if this is a change in breeding, or if my Mother just didn’t cook them long enough.
Wendy’s Frosties and fries. When I was a kid, the frosties had little flecks of chocolate in them. It was almost like that magic shell type of chocolate, but broken up into flecks and mixed in. Also, the fries at Wendy’s used to be thick steak fries. I was saddened when they went with the same shoestring fries everyone else had.
When was that? I absolutely believe it, but Wendy’s has has non-steak fries as long as I can remember (switched to the ones with skin on them sometime in the last 15 years) and frosties did not have chocolate flecks when I was a child. My memories probably only go back to mid 80s, but late '70s commercials on YouTube don’t have steak fries, either (not sure they are skinny enough to call shoestring, but long, thin ones, not wide ones - smaller than crinkle-cut, anyway). Though it could have varied by location.
All of them. As I’m getting older, my sense if taste is blatantly weakening. Everything has to be over seasoned for me to taste it much. So I don’t know for sure which foods have actually changed their recipes, which are just the result of my fading taste buds, and which are reality not matching up to nostalgia. A case in point is that as a child my favorite food was probably KFC original recipe chicken. Now? Bland. I have to slather it in spices of my own. No idea if the recipe has changed in the intervening 35 years or so.
It was the mid to late '70s. I’ve been looking for evidence for years but I’ve yet to find it.
I remember Wendy’s since the late 1970’s; they never had flecks of chocolate that I can recollect. They also never had vanilla Frosties, which is apparently a thing now.
Wendy’s fries used to be the best in the industry. Now they suck. But so do everyone’s. I need to go back to Europe and have tallow-fried fries.
Peaches. It’s so hard to find a good peach now, but they seemed commonplace when I was a kid. (30 years ago or so)
I haven’t noticed any change in pop tarts and I went about 20 years without eating one.
Burger King fries are way better than they used to be.
Olive Garden food is terrible now. Used to be pretty solid, but the chicken is awful and the breadsticks are worthless.
Chili’s used to have these really good chips they deep-fried themselves and came out fresh in interesting rose-petal like patterns. Now they’re just boring old bagged chips like Tostitos.
Cucumbers! Why are most cucumbers squishy now? They used to all be firm and crisp, didn’t they?
Gotta go back farther than that, youngster. 
Go back to the 60s and 70s.
Hostess cupcakes.
They used to have much more filling, and of course they were bigger. But the icing is more plastic-y tasting. And Twinkies are bad now. The little Debbie ones are better. And while I am bitching, why did Hershey change the wrapper on a Hershey bar? It was so satisfying to open the 2 part wrapper. Even the miniatures had 2 wrappers. I even miss the foil on juicy fruit gum. Ok, rant over.
Don’t buy regular cucumbers. They are nothing but water.
Buy Kirby cukes. Smaller, denser, firmer. If you are following a salad recipe, say, use two Kirbys to one regular.
If Kirbys are unavailable, look for Persian cucumbers. Not as economical as Kirbys, but still much better than those bloated fat zeppelins sold as “cucumbers.”
When I was a kid, my favorite candy bar was Cherry Mash, but I rarely got one because they cost more than normal candy bars. A while back, I saw a Cherry Mash in a store and excitedly purchased it. The pink stuff inside was really nasty. I have to believe the recipe changed because I couldn’t possibly have ever liked that crap.
Hostess fruit pies. No comparison.
McDonald’s fruit pies. They were crusted with something akin to Captain Crunch and deep fried.
Apples, specifically Red Delicious. While breeders have made great strides in creating a perfect palm-sized treat that can be stored for months and shipped thousands of miles without bruising, they’ve also bred both texture and flavor out of it.
Banana cream filling. I don’t think Hostess ever used it, but one of their competitors (Dolly Madison?) did.
Asparagus. That stuff my mom made out of the can was awful, but the stuff I buy in the produce section of the grocery store and saute/grill until just al dente is really good!!