*Lay’s Caribean Style BBQ chips. They were far too good for mortal man.
*Kellogs Special K, plus protein edition. Its prescribed by my nutritionist, but the Shopright & Pathmark by me don’t carry it anymore. Whenever I see it anywhere else, I buy out the shelf.
*Lockes/Kilbeggan Irish whiskey. Lightyears ahead of Bushmills or Jameson; better than Powers (and that says something). Almost worth the airfare, if the b-stids at customs would let you bring more than 2 bottles back. grrrrrr
These are probably because I’ve moved down South from Massachusetts, but I really miss Cain’s mayonnaise and Habitant Split Pea soup (This is actually from Campbell’s of Canada).
I miss Kellogg’s Honey Crunch Corn Flakes. I haven’t seen them in stores for a couple of years. The linked page is for Kellog’s Canada, so maybe they just sell them there now.
Oh yes I used to love this Lean Cuisine too. I haven’t seen them lately but didn’t realize they were gone for good. They still have them on the LC web site though: Asian-Style Pot Stickers
Also, I love these Crest toothpastes, but I can rarely find them in most grocery stores and drug stores: Crest Plus Scope Minty Fresh Liquid Gel and Cavity Protection Cool Mint Gel. I will see the white Scope liquid toothpaste and the paste version of the “cavity protection” toothpaste, but never see the mint gel versions!! I just don’t understand it.
Just a couple years ago, Quaker Oats introduced this yummy oatmeal bar. I don’t remember the name of it, but they now have something called Breakfast Squares (or maybe it’s Oatmeal to Go?), which is vile by comparison. They weren’t a snack bar, or granola-bar type thingie – more of a Bowl of Oatmeal Replacer. 
Anyway, I loved them, ate way too many of them, and mourn their demise.
Also, I haven’t seen Vidal Sasson shampoo on sale for a long time. I used to love it, and have wanted to go back to it. Can’t find it.
Heluva Good Clam Dip - I mean, they used to make it and I think they still do, but every store in the south carries only French Onion and Bacon and Horseradish. I haven’t even HAD dip in ten years, but I would break that rule in a heartbeat for some yummy clam goodness.
Similarly, Humpty Dumpy Sour Cream n’ Clam Potato Chips used to be a favorite of mine growing up in Maine. Having long left the state, I discovered that I was able to buy them (and their similarly awesome BBQ) online by the case. Unfortunately I ordered only one case of each before they got bought out by the damn French Canadians. Now this flavor has been discontinued, their website is never updated, and you cannot even order their stuff in reasonable quantities online anymore. 
Yes it annoys me, because I always like the taste of the flavor that makes it as a product line, but dies in a year. I go to the shelves and find 5 flavors, just not mine, and the website tells you they are dropped. I had to not make at least 3 different Christmas dishes this year, because a main ingredient wasn’t made any longer.
Hershey’s soft carmels
Mrs Grass onion and mushroom soup mix - I used it in a lot of stuff.
Betty Crocker Ranch potatoes and the bacon an cheese ones.
Many I can’t think of off the top of my head.
Licorice Altoids. I had them once and then they disappeared.
China Lily Soy Sauce - I had heard a few years ago that people were having problems finding it, but I never couldn’t find it and then I heard that everything was running again. Now I can’t find it anywhere, but I’m not hearing that from others.
Mitchell’s Honey of a Garlic sausage - Safeway suddenly stopped carrying it, and it’s not at any store I can get to. Don’t know what happened to it.
Peanut Satay sauce - can’t seem to find it anywhere. At least not the one I liked (I forget the brand) I bought one bottle and then it disappeared. Now the only one I can find is a version that tastes like stale peanut butter and nothing else.
Yep, I remember when they changed them from good to inedible.
For me it’s Eclipse Coffee Syrup which I lost when I moved away from New England and was then bought out by Autocrat and changed in a way that made it much inferior to the original.
The two products I can think of off the top of my head are:
Old Bay Seasoning and this Hickory seasoning stuff. I cannot find it anywhere around here. I do not know why.
I can probably order on-line, but it seems ridiculous to pay that much for it.
Yes!!! My husband and I were lamenting the unavailability of these just the other day.
Trader Joes markets carry these.
I just bought a big tin of it the other day at the grocery store.
Maybe I can hook you guys up 
I just remembered another one. About 8 or 9 years ago, I was using a shampoo called **Real Shiny Hair ** and it smelled like an orange creamcicle. I haven’t seen it in about 8 years.
I don’t know that they are gone forever, but I know that none of the stores in my area are carrying them anymore. I should ask a manager at the stores.
LifeSavers Swirl Pops were my favorite candy until they vanished. And my whole family loved Blue Bell’s “Chocolate Almond Marshmallow” ice cream, but I guess it got to be too expensive to make and was pulled from production.
Texas Gunpowder,made from ground jalapenos.
We ain’t got your fancy trendy supermarkets down heyar.
St. Ives Whipped Silk body lotion. I loved the stuff. It was lighter weight than their regular lotions. I believe they quit making it.
Can be found in the impulse-check out lines at Meijer. If that helps.
I, too, rarely see Licorice Altoids. When I do, I buy the entire case. They are most excellent in subduing a cough if you let them dissolve in your mouth. (natural licorice, not the red candy stickie things, is a natural cough suppressant.)
I haven’t seen the Ginger Altoids in a while, but they were excellent for dealing with a barfy stomach situation I had two years ago.
While we are on Altoids, have you seen the new Dark Chocolate Dipped Cinnamons and Peppermint(?). I wuvs the cinammons.
Things I have a hard time finding Plain pop tarts. I don’t want the frosting. It is now down to maybe strawberry or one other berry kind.
This isn’t a supermarket thing and it could be just a regional thing:Towne Club Soda
God, I miss going in there with a twenty four pack of empty bottles in the case and picking out 24 new and exciting flavors.
Whatever happened to Towne Club Soda?
For a dismal period a year or two ago I found myself completely unable to locate any metric graph paper (5 mm). I love this stuff for my various weird interests and was distraught at being unable to find any. When I finally did find some I bought ten notebooks.
Tropicana used to have dragonfruit juice in those “Twister” mixes. I would buy it for my kids when they were younger, and they would polish off the bottle in 10 minutes. I started buying three, then five at a time. I really liked it, too. So did a lot of other people— they were always out of it at the grocery store. Then all of a sudden, it just stopped getting restocked, and the gap on the shelves was filled with some other brand. It was a lot like the ribwich on The Simpsons.