For me its the ‘Knock Code’ that LG had on their phones (I really wish LG still made at least the V series phones)
Basically there was a four-square area and you set up a sequence of where you would tap to unlock the phone. That set of squares was only shown when you set up the code
Then, to unlock your phone, you would tap those areas in the sequence you set up (even with the screen off).
Fingerprint readers are nice, but I really do miss the knock code
Edit: did find this article with a way to do the knock code, but if done wrong, could brick your phone I guess.
Plus, article is from 2014. When I looked at XDA’s info on it (they also being the developers) it looks like development on it is over, but individual modules may or may not still be supported by their devs
There were a couple of phones HTC made (both under their name and rebadged as early Google Pixel phones) that could detect squeezing the phone as a programmable button press. It seems like it’d be clunky and triggering at the wrong time or not triggering reliably when you needed it but it was just really well implemented so it worked perfectly. Slightly increasing how tightly you’re holding the phone is such a tiny thing to do so getting a full extra programmable button out of it was actually really useful for making your day to day phone usage slightly smoother and more efficient.
I’m guessing it just didn’t get enough use because people aren’t likely to try it intuitively.
I relatively recently upgraded from a Pixel 6 and constantly try to use the squeeze feature that doesn’t exist on my new device.
It just is more useful to have it than not (especially as it can be easily adjusted or even entirely disabled for those who don’t wish to use it. Iirc you have to intentionally set it up during initial setup of the device.) It’s a shame not to see it widely adopted.