The 3.5mm audio jack. It's so fundamentally simplistic from a manufacturing standpoint and circuitry standpoint that any headset you throw at it will work identically without fail (the key innovation being the speakers or headphones where the analog signal is sent to).

The Bic pen. Sure, you can make it better, but then the price has to go up. You can still buy a nearly unchanged Bic pen from any office store for cheaper than any other writing tool, nearly identical to what they looked like when they were first invented.

Potato peelers. The ergonomic handle was a big step forward, yes. But the basic design hasn't (and likely won't) change.

The MIDI protocal. The technologies that use it have evolved in all sorts of ways, but the protocol has remained unchanged.

I think sewing machines would count? They certainly got a hell lot more "portable", but the basic design hasn't changed much since the 1880s. Those things are little mechanical marvels

Microwave oven. It sort of just...appeared, and the design didn't change much.

Dinner plates. Wooden, marble, ceramic or whatever it's made from, it does it's job perfectly.

EDIT: Yes, I'm hungry

Solid body electric guitars- the first models have been in continuous production and are still available.

https://en.wikipedia.org/wiki/Fender_Telecaster

https://en.wikipedia.org/wiki/Gibson_Les_Paul

There were earlier "electric guitars" but I'm thinking all inventions build on previous creations. I don't think you'll find many pure answers to OPs question. I think the closest you'll find is going to be an advancement that produced a single step change in design that flattened the innovation curve forever after. I think the microwave oven was a great example.

Electric fuses also come to mind. Little has changed since 1890.

ramps

deep-frying

the D-pad

Velcro? Inspired by nature's invention

https://www.microphotonics.com/biomimicry-burr-invention-velcro/

Also outdoor grills don't seem to have changed much other than the material used to keep the fire going.

RSS Feeds

Paper clips.

🧷 Safety pin. There has been a little change in the safety cap but that's to save material not functionality or manufacturing.

The entire process is the same:

1. Take wire, cut it
2. Smash one end flat
3. ?? (Bend the wire and fold the smashed end)
4. Profit

I don't think QR codes have changed at all. Only the tools we use to scan them have

On a high level, all simple machines.

The wheel

The lever

The pulley

Etc.

All other machines (except maybe things like computer chips) are just a variation of simple machines, or a combination of them.

The bowl.

The pointed stick.

Lego. Lego from now will still mate with Lego from 40 years ago without a problem. Apart from a growing number of shapes, the basic blocks are still the foundation of everything sold today.

Maybe FM synthesis, it revolutionised the sound of the 1980s and music production as a whole

The headphone jack.

It's very niche, but the only thing I could come up with is Kvevri, a traditional Georgian winemaking vessel. They're sold today (and still used for their stated purpose, aging wine), I've personally seen kvevris with the exact same shape buried in a wine cellar of 12th century monastery, and at least going by the article they're like 8000 years old, and haven't changed much in that time.

My other ideas were:

• Bricks (turns out the earliest sun-dried mudbricks, which are very different from modern ones)
• Concrete (turns out it changed a whole lot since the Romans, modern concrete is much easier to pour, sets faster and is much stronger)
• Nuts & bolts (initially were hand-crafted and non-interchangeable - yuck!)
• Knives (I'll let knife enthusiasts speak about that one)

[off topic?]

I can't remember the exact quote, but Robert A. Heinlein said of the DC-3 that it was the best airplane ever built, and that the only way to improve it was to completely redesign it.

I just like the idea that some things are perfect the way they are.