Telling even the newer devices about your pubkey doesn't get the scare screen away. You see a Yellow Verified Boot warning meaning the OS is signed and verifies

Telling even the newer devices about your pubkey doesn't get the scare screen away. You see a Yellow Verified Boot warning meaning the OS is signed and verifies but with a custom set of keys. When you lock the bootloader you block other keys, since fastboot is pretty much disabled when you do that, and the only way to install something would be via OTA updates which would have to be signed with your custom keys. I guess maybe if you're able to get a root exploit and replace the boot image? Not exactly sure what would happen then, need to try.