A built-in scripting language. The TI-83 line of calculators have an app programming language that requires you to side-load code from another computer, but they also have TI-BASIC, which allows you to write a wide variety of scripts right on the calculator itself. This should be standard on all 'smart' devices. It's so stupid to have gigahertz of computing power in your pocket and not be able to do anything without writing the app on another machine.

I know Termux for Android exists and that's a good start, but I'd like to see something baked right into the OS that has access to all my device's cool sensors and gizmos. The camera, the microphone, the aux port, the usb port, the accelerometer, the bluetooth antenna... all of those things should be exposed to the user. This would be a really good use case for 'visual' programming ala Scratch, since you could assemble a script right from a touch screen instead of having to plug in a keyboard.

My 2001 flip phone could schedule sms messages to be sent and it took smartphones a decade ish to add that feature.

Buttons.

ITT are many middle aged nerds

IR blaster.

Ad-Hoc wireless networking. I miss it, was useful back in the day if you needed to share files with multiple people without a wireless router at a location. Most laptops don't support this anymore. To be fair, I've only really wanted to use it maybe twice in the last 10 years.

Lack of notch

I miss tether points. We have these super expensive, slippery devices and we have to stick something like a pop socket onto them to be able to get a good grip on them. I used to have these little dangly thumb loops that if I dropped my phone, it would just dangle there instead of slamming into the ground. It's very minor, but I don't understand why they don't have them anymore.

My phone is 4 years old and this sounds like a horror story to me. I have no idea what I'm going to do next.

Hardware alarm so I could turn off my phone at night...

Replaceable battery, micro SD card slots are hard to come by these days, too.

Durability, battery life, repairability.

Printers that work.

Pixel phones and I believe Samsung phones just light up the OLED display to let you know that there is notification. An independent LED was only necessary because screens would have to light up the whole display to indicate notification,but now we have better screens so that isn't necessary.

What happened to the notification light?

My Orbic Myra 5G has one, and I got it for free.

I want a IR blaster again and a front notification light.

Or at least let me easily simulate the notification light without a third part app.

The rotary disc on phones!

I miss the home phisical button and back/menu touch 'button' on tablet and phones. Having to swipe down from the top, then press the right symbol at the bottom before they disappear again is a mess.

Physical buttons.

I still require all my phones to have a SD card slot since the higher capacity versions are way more expensive than even a UHS card.

Yes it's getting harder to find newer phones that have it above entry level models.

That crackling sound you had on your calls back in the analogic era. Not sure is a feature but made calls more warm.