There's no reason why it can't be supported. ROMs like Graphene have been supporting longer passwords for ages so seems like some arbitrary Google restriction. Yet another reason why I stay degoogled.
I learned it from another poster. I did the brightness trick when when I setup my fingerprintes (you setup four finger prints with the same finger, but with each fingerprint setup you change the brightness by %25 step. you start with the lowest brightness [assume it's 0%] to the highest [%100] with each setup).
Thanks to that trick, I had at least %80 unlock success rate with my previous pixel 8P Pro!
I upgraded to the P9 Pro with the ultrasonic sensor, and unfortunately it's actually worse than the optical for me, and add to that brightness trick doesn't work with it either since it uses a different tech...
Biometrics are passwords you can't change. Depends on the implementation and hashing, but the digitization of your body is still just a complex number that maths through the ALU in a compare registers operation. That number can be replicated on some level, but you cannot change that number if it is lost.
I use fingerprint authentication for my phone on The Daily, I will never use this feature. Strictly because the fact that fingerprint sensing is so inconsistent and any type of change to the phone screen requires you to completely redo your fingerprints. Not to mention any type of substance on your hand suddenly makes you unable to use the phone, just when swimming for a half hour enjoy not accessing your phone just took a shower yep you're going to wait an hour before using your phone.
Fingerprint authentication is super handy and I do like it, but I would never enable A system that required the fingerprint in order to pass the login