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
Plenty of phones have unlockable bootloaders and it used to be pretty much an expected thing on Android phones until manufacturers and carriers started locking it down and being more Apple-like. You can’t run most custom ROMs without an unlocked bootloader as being able to run a custom kernel requires an unlocked bootloader. Being able to use non-Android Linux operating systems like postmarketOS also depend on unlocked bootloaders.
On most it’s just a matter of toggling an option in developer settings and using fastboot to unlock. Some make it more difficult than that, others completely prevent unlocking (and thus become e-waste after the official software stops getting updated).