I never forget to turn off my phone, probably because I'm so used to pushing the off button when I lay it down because I disabled auto lock on my first iOS device (1st gen iPod Touch). Maybe it's better now (by registering attention via face id), but back then I often found myself keeping something open as a reference and then having the screen turn off when I needed it.

edit: I just checked and indeed the settings screen says it watches for attention, so I'm willing to give it a try and enable auto unlock now...