YoYos. Still around, but I haven’t seen one in use for ages.
Hats. I wear a hat when I’m out more often than not. I see very few other people wearing one.
my impression was that hats had been popular rather than what the OP is looking for.
Yes, I thought that this thread was about what has both appeared and then disappeared in your lifetime, not just what has disappeared. Hats, yoyos, and prostitutes were around long before any of us were born.
My example would be Earth shoes. That ugly, weird footwear that had a sole that was thinner at the heel than the toe, supposedly better for your posture and overall well-being. Extremely popular for a few years in the 70s, they seemed to have completely vanished.
How old are you that Sears became ubiquitous in your life time? 
Also, telephones that were only telephones.
I am 3 years retired now, but I used both fax and pager literally every working day.
My ex cow-orkers still do.
mmm
Dollar stores (the kind that sold a wide variety of merchandise, all priced at exactly $1)
Yeah, now they’re $5 stores.
I’m 61, and I remember when pretty much every mall in existence had Sears as one of their anchor stores.
Hey, speaking of malls…
ETA: oh, I get it— became ubiquitous within one’s lifetime, not already ubiquitous. Never mind 
As others have said, the video store industry rose and fell during my adult lifetime.
I bought my first vcr around 1983. At the time there were only two stores within an hour’s driving distance where you could rent vhs movies. I think one of them also rented laser discs, which were already on their way out.
Within ten years there were videostores everywhere. Pretty much every town had one and most had several. And in addition to stores that specialized in video rentals, many other stores rented videos as a sideline.
Now there isn’t a single video store left in western New York. The last videostore in my area was a Family Video that closed in 2021. The closest one still in business is a four hour drive away.
How about disposable digital cameras?
Another I thought of were basketball shoes with embedded air pockets, which you could inflate by repeatedly pressing a button.
There was a time in the 1990s when you’d see NBA “pump up” their shoes before the dunk contest at the All-Star game.
Again some of the older people here might remember it: only a very small percent had black and white TV in 1950 while the substantial majority had it by 1960. So it’s possible some people here remember listening to radio for news and for various dramas and comedies–and then remember the big event when their family first got a TV.
And the ones that are still around are all but dead.
I went into Brookfield Square mall a while back. At one time a robust, bustling center
It was eerily quiet, more stores closed than open. Like a ghost town where something terrible had happened before everyone left. Creepy. That’s the word to describe it: creepy. Like a lost episode of the Twilight Zone.
I picked up my shirt at the JC Penney (I was the only customer in there) and skedaddled.
Dial-up modems! beep beep bee bee booo… brrweee-eeeeeew-eee-oo, ee-oo… crrrrrrrr
Shoot, a fella could have a pretty good weekend in Vegas with all that stuff.
I don’t remember these at all.After reading about them some, I don’t think they were ubiquitous - they seem to have been introduced somewhere around 2003 and I’m pretty sure I remember cell phone cameras already being common by then.
this is a good one. By the time I started getting around to thinking about replacing incandescent with compact fluorescent bulbs, they were already getting hard to find, so I essentially just went from incandescent to LED directly
I’ve not seen leds here yet. At least I haven’t seen leds that would go in a table lamp or overhead fixture. I see lots of compact fluorescents in brick and morter. I’m gonna’ keep an eye out for those.
Where is “here”?
Discos. That type of “dance hall” became hugely popular in the late 70s, and seems to have disappeared completely in the US.
I probably still have my “Members Only” jacket in a closet somewhere.
Also, T-Tops in cars. I really liked those, as a good compromise between a convertible and a hard top. I suppose they’ve been replaced by the even better hard-top convertibles.
Showing my age even more: CB radios. They were everywhere, but seem to have disappeared. Maybe truck-drivers still use them, but I haven’t seen one in decades.