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.
S10e was still one of my favourite phones. Right size, waterproof good fingerprint scanner placement. And a headphone jack! Now I've upgraded to an S22 which isn't terrible, but it's missing the headphone jack.
This is one of the big reasons I got a Galaxy Fold instead of one of the flagship slab phones, which all have under display readers at this point. It's so much smoother.
I had a Flip 4 and an S10e before and I have a S23 now and I wish still had the side key fingerprint, the inscreen scanner often misreads my thumb for some reason.