What old technology are you surprised is still in use today?

Fax machines!

They were invented long before the computer or modem, with the original patent being issued in 1843.

They seem wildly outdated, but the ability to replicate the signature (iirc) led to faxes being accepted as legal documents.

This speaks more to the underlying usefulness in earlier eras, but it's still wild to make a phone call that leads to a printed document.

Faxes are secure after sending. The only way for a third party to get a fax is to physically tap the line at the time of transmission, or to break into the building and steal the paper.
Email sits online, where everyone can get at it unless you prevent it. Faxes sit in a drawer, behind the locks you already have.
At my old government job, we had a fax machine because it "couldn't be hacked", so we would only accept certain documents via fax. Is this true? I have no idea. It was even more questionable when we switched the fax line to digital and received all the faxes via a software program on computers. Is THAT "un-hackable" too? No clue.

An easy answer is pagers in hospitals. I know it's because they're simple RF technology, and work reliably in cinderblock buildings. But given how advanced so much of our medical equipment is, you'd think there would be a different system. Granted, that system would almost definitely need updates and have potential downtime/crashes, which you cannot afford in that kind of environment

Some telcos are willing to guarantee that pager messages will arrive. No-one's willing to guarantee shit for mobile phones.
Modern hospitals have an integrated wifi based network that mimics the cell system.
- The one I worked at was a very large modern university hospital (in the US) that handled a large part of the states patients. We used pagers in the Emergency Room for code red patients. Many doctors also still use them
There was a good episode of Planet Money where they interviewed some doctors who were trying to get rid of beepers at their hospital. Basically during testing, it was found that when using an app to send messages to doctors, it was almost too simple and non critical messages were being sent to doctors. This was creating a lot of noise and causing them to be overwhelmed and ignoring the notifications on the phone. It seemed as though the clunkyness of paging someone was a feature so that doctors on call were actually getting relevant information.

Cars.

Steam.

Always impressed that a nuclear power plant actually, doesn't use much nuclear physics/engineering but is mostly a steam machine (with a top notch turbine)

Thought you were a PlayStation fanboy for a second.

Clipboards with pen and paper at the doctor’s office check-in visits

As a programmer it makes me livid being forced to poorly hand fill the paper only to see someone at the desk enter it all into a computer.
Also, couldn’t I have done this online beforehand?
- You forgot the n^th generation copy of a copy of a copy that has no true blacks, just these weird grey pointillism bands that are so askew your basically writing at an angle.
  I also have some horrendous carpal tunnel syndrome, if I’m filling out something on a clipboard, my already rusty handwriting is going to rapidly degrade even further.
- I’ve literally asked them this, and they just shrug apologetically. It is what it is, is the attitude I am met with.

I'm not surprised at all since I use them every single day, but the pen and paper have yet to be rivaled by anything digital. At the very least, in regards to:

portability: available in any size one may fancy, only a few grams.
autonomy: a notebook needs no battery and offers weeks or months worth of storage depending how much you write or sketch (certain models can even be refilled),
ease of use: put pen(cil) onto paper and write, or sketch + no upgrades, no bugs, no crashes.
privacy: no tracking, no spying by any corporation. No ads, either.
low cost: I've yet to find an iPad with its Pencil at the two dollars I spend on my cheap notebook and cheap ballpoint, or pencil.
Sturdiness: I can (and often) sit on the notebook I store in my jeans back pocket, I often use it under the rain too.
Low attraction to thieves: I can use it anywhere without risking attracting attention from potential thieves. I can also let my notebook alone in any public places, chances are no one will even consider stealing it: it's just paper. Try doing that with a digital notebook, be it a tablet or a phone ;)
Versatility: I can write (errands, novels, plan to conquer the world, a poem for my spouse), sketch (bad or good sketches), draw a map, play some games, make paper planes or origami, and even share info with anyone by tearing of a sheet of paper from my notebook (that won't break it) and give it to that person.

Yeah, I think it's kinda obvious I do like my cheap notebook and pen, a lot more than I will ever like that corporate and government spyware that disguises itself as a smartphone and that I'm expected to be using and carrying with me everywhere I go ;)

Toilet paper

Fax. Its all over germany still the standard in the bureaucracy

When I started a new job in IT around 2009, I learned that AS400 / iSeries / IBM i was a thing, and it's still doing pretty well with big retail and the insurance business.

In the same vein, the system used by the aviation industry to book flights is also quite ancient but still very much used to this day.