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
Never been a fan of rear fingerprint scanners, always make unlocking a phone one handed when it’s on a desk at work or whatever so much more annoying.
I mostly used the Xperia Z line until the pixel 3. And at the time when I was looking for a replacement to my Z5 which had a power button scanner on the side. Nobody, not even Sony at the time were doing them since they buckled to then start mostly copying samsungs design language. And now that phone companies have finally realised how good a power button scanner can be I’m stuck with the pixel 3 haha. I bet by the time I need to replace this some other feature in phones I love will be mostly absent, knowing my luck.