My 2014 Mazda 3 stick/manual transmission has cruise control on it but, yeah, it won’t down- or up-shift, obviously. But when you’re on the highway in 6th and just want to maintain speed for a long period of time, it’s fine. I live in the Midwest, so it’s not like there’s a lot of hills or reasons for constant gear changes.
It’s pretty handy, although occasionally people nod off: https://www.youtube.com/watch?v=LcMOtjE50XQ
I recently bought a new Toyota Camry. The first time I used cruise control I didn’t know about the adaptive part. I thought it was incredible! Very cool invention. And I like the “stay-in-lane” feature as well.
It did happen on The Simpsons. (Skip to .35)
I have personally approximated that stunt and could easily perform it. Context is critical. I took a previous 25-foot RV out to the middle a a large, flat, Nevada dry lake bed. I set cruise control to 25 MPH and released the steering wheel. We gently looped around, far from any obstructions. I could easily, and probably safely, have stepped back to the kitchen area to brew a pot. But I’m rather more cautious. 
Don’t try this on the freeway, kids.
The father of one of my friends lost attention/fell asleep at the wheel, and drifted off the (rural) road. Drawing on a lifetime of driving experience, rather than jamming on the brakes and spinning out of control, he took his foot off the pedal and waited for the car to slow down … and it plowed through the tall grass, the hedge, up the rising stone of the gate, and head-ended straight into the ground as it came off of the wall.
Fortunately, being a reasonably new car at the time, it was also equipped with air bags. He wound up with two black eyes from the sudden stop. That was a recent enough innovation that the tow operator thought it had been a fatality for sure when he saw the car.
It wasn’t always so. The old American cars with cruise control were 3 speed automatics, but that also meant that the engine was tuned with a wide flat curve: you didn’t need to change gear as you slowed down or speed up: the other two gears were just for starting from a stop.
The bottom line was that if you had a simple cruise control controlled by inlet manifold vacuum, it did alright in a big old American car, automatic or manual. In a modern car, cruise control needs to be designed to match the automatic, and the automatic needs to be designed to match the cruise control.
When you put the same kind of cruise control into a modern 6 speed automatic, it is crap. It can’t hold the speed constant enough to stay in one gear. And the sharp tuning of a modern fuel-efficient engine, tuned for a 6 speed gearbox, means that the speed rises and falls sharply. And then the automatic gearbox (which is not connected to an old style cruise control) shifts, and the cruise control is suddenly out of wack, you speed drifts even more, then catches up, whatever and so on.
And a six-speed manual gearbox in a cheap modern car just doesn’t work well even if you don’t have cruise control. You have to be churning through the gears, cruise control or not. A 6+ speed gearbox in an expensive car works well only because (a) the driver wants to work the gears and (b) the engine has so much power it can drive in any gear.
I had to look back to see if this question was asked here in 2020.
Seriously OP, from England and you don’t know what cruise control is?
I follow, Nansbread, but there are plenty of urbanites around the world who have never learned to drive. That’s true of plenty of New Yorkers, Londoners, Parisians and Dutch.
So I can imagine that someone who never learned to drive (or has a license but hasn’t driven in years) wouldn’t know what cruise control is.
I would expect such a person to find the answer with a single Google search, but asking here seems reasonable to me as well.
The last three cars I’ve owned were all 6-speed manuals with cruise control. The first two only controlled the throttle, so it was possible to exceed the setpoint speed when driving down a steep hill in a high gear. My current car brings the brakes into the mix to prevent any downhill runaway.
Often there are restrictions as to when the car will let you use cruise control, e.g. must be going at least 30 mph in 3rd gear or higher.
I’m in the traffic-choked UK so I don’t get the chance to use cruise control much. I do, however, use the speed limiter function all the time, which uses the same hardware systems as cruise control but with different firmware routines. It really takes the stress out of a journey that’s liberally sprinkled with speed cameras. The legal tolerance on UK vehicle speedometers is -0/+10%, which means that the indicated speed will always read a little high. On new-ish tyres inflated to spec my current car reads 2 mph high at 30 mph, and 3 mph high at 70 mph (as calibrated by a variety of sat-navs on a long straight), so when I want to be doing exactly 30 I dial in 32, etc.