Android

Easily notification light. People always say "oh, it's totally obsolete with always on displays". But with a notification light I could focus on other stuff and the blinking light got my attention better. With the AOD, I always catch myself glancing at my phone. Also, the light's color clearly indicated which app caused the notification. I had White for calls, Green for Whatsapp, Yellow for the ebay app, Red for GMail and so on. "You can do all that with an OLED screen! It only lights up the pixels that-" Can you, though? All apps that I tried were utter garbage. Buggy performance, very battery hungry and very cumbersome to configure. I don't know if custom firmwares actually have that feature in a usable state nowadays, as I cannot root my phone anymore without losing core functionalities like online banking.

Yeah, everything tends to go to shit with time. I miss my Galaxy S2.

Does IR sensor count? I loved that on older phones so much.

Not so much a gimmick, as much as something that seemingly went extinct that I miss: rear fingerprint sensors. I loved them on my Nexus/Pixels, and the in-screen one on my 6a is way less consistent and convenient.

Also it flashbangs me when I try to unlock my phone at night.

Apparently nearly everything I look for in a phone. Others have said IR blaster, side squeeze, notification light, and pop-up front camera, all of which were amazing.

I'd add an unlocked bootloader (I bought it, it's my phone to do what I want with), removable battery (hello instant charging), and a small form factor (so sick of needing two hands to do anything).

Edit: Forgot one, on the back fingerprint reader.

Power button fingerprint sensors. I had one on my S10e, and I loved it - with the way I held the phone, my thumb naturally rested on the power button, so it was pretty much auto-unlocked.

Now they seem to have fallen by the wayside in favor of in-screen sensors - which are cool, but ever-so-slightly more cumbersome. Ah well, still better than facial recognition.

Fingerprint sensors

FM radio. Also my old Motorola had a "karate-chop to activate camera" which was very useful

Idk if this is a gimmick but I love swiping on the rear fingerprint scanner to pull up/down the notifications and quick settings. I also got an app that lets me swipe left/right on the sensor to adjust the brightness.

That pop up camera on the OnePlus 7 pro.

That thing was cool as fuck. My roommate got the phone and I was VERY jealous even though I had a OnePlus 8T at the time.

Removable battery, micro SD, sim slot etc

PIE navigation control

Nokia N95 flipping both up and down. I really liked those music player buttons when flipping it down

On my motorola: quick shake side to side to enable flashlight. So easy to use, it's become second nature. I'll have to find a way to replicate that on the next phone I get.

Material You. I wondered why they wasted resources for ... colors. But it's so nice to have a consistently colored UI across apps and across dark/light modes, and I wished that more apps would support it. Also, those pastel colors are less stressful for the eyes than the previous grey/blue.

I know it's not everyone's taste but I really like it.