Trader Joe’s “discos” products I love all the time. I learned early from them that “disco” means “discontinued”.
My most lamented is their black pepper cashews. I know other companies make them, but they’re not the same!
That Dove White Ginger & Nectarine shower wash. Got two bottles in and bam! Gone. I haven’t found a single other peach or nectarine scented bodywash on the market at all, and it is slightly upsetting.
Crispy M&M’s as well.
I’m sure a dozen other things I’ve managed to forget about. Easier that way than pining forever, y’know?
I’ve learned my lesson and if I find something I REALLY LIKE I will buy several of it. I still have another pair of my favorite sandals waiting in the wings for when my current pair give out.
If a store stocks one item from a certain brand but not another, 9 times out of 10 they can, they’re just choosing not to (regional choices at the manufacturer level aside). Write the store manager and tell them you’d like it. I’ve had several food items suddenly appear back on the grocery store shelves after a nice letter.
From my childhood (I don’t think I’ve seen one since the early 60’s)…
Pineapple flavor Tootsie-Roll Pops
Buffalo Wild Wings used to have a sauce called Smokey Southwestern. It was my absolute favorite! Great flavor, not one of the stupidly-hot sauces aimed at drunk college students.
So of course they discontinued it. And there really isn’t any other sauce like it. There’s no point in me going to Buffalo Wild Wings anymore. 
Oh lord, that reminds me of their curry sauce. I LOVED THAT and they discontinued it.
Hydrox cookies.
I hadn’t thought about those for a while, but now thanks to you I miss them all over again. They were good.
Gone forever:
Archer Farms Honey Almond yogurt.
Almay lipstick called Pinkcopper. It wore beautifully and was a perfect color for me.
Trader Joe’s Garlic Simmer Sauce
Total Cranberry Crunch cereal
Grapenuts O’s
“New and improved” and therefore not the thing that I like anymore:
Olay tinted moisturizer.
A different Olay Total Effects eye cream.
Any number of hair products from Prell to Pantene
The Hanes cushiony cotton socks I liked.
Nabisco Sugar Wafers (Biscos)
Chef Boyardee Beefaroni
Vault Zero. My wife loved that stuff. It didn’t stay in the stores long… (Wikipedia says it was sold 2005-2011, but around here it was more like 2005-2006.)
When I get a time machine, I’m going to go back and buy a few cases of Seven Up candy bars.
Apparently those were never carried where I shop. I would have loved that. As it is, I buy the regular ones and dip them in cherry or blackberry preserves. 
Oh, and Gatorgum. (made by the Gatorade folks.)
Too many to mention, but here are a few:
Tostitos Lime and Chili flavor
Kitty Clover BBQ potato chips. Actually all the Kitty Clover chips, but the BBQ were the best.
Hiland potato chips, doesn’t matter what flavor.
Crest used to make a lemon flavored toothpaste. As I hate the taste of mint and love lemons, this was perfect. Several years ago it slowly started disappearing from every store I went to. I did stock up on a couple tubes before they were all gone. I squeezed the last little bit out of that last tube.
If you mean the chocolate-covered caramel twisty things, I think Cadbury still makes its version (Curly Wurly), if you know where to look. (“Marathon” is also what Snickers were once called in England - IIRC, at one time, Marathon was really Snickers, Snickers was really Milky Way, and Milky Way was really 3 Musketeers.)
My two:
Original Product 19 cereal - a few years ago, it was reformulated into something that tasted like Special K.
Canada Dry Cactus Cooler - I haven’t found a decent orange-pineapple soda (carbonated or otherwise) in decades.
I’m not even going to mention what my favorite dental floss is because I know They will discontinue it the moment they find out it’s the only one I use.
Cruncheroos.
They still make new balance shoes! I just bought a pair last week!
And they canned the Black Forest Tim Tams too…I could go through a whole packet in no time flat, but alas, no more. The Originals (and even the dark) just don’t cut it for me anymore.
Bastards. 
I liked it too, but it was three and a half times the price of “normal” Coke, and was always doomed at that price.
They’re back, around here anyway.
New Coke. That stuff was awesome.