To disable the user inputs at u-boot, boot loader and Junos Kernel boot stage, use the chassis routing-engine bios command as following:
On SRX Series devices, if both set system ports console insecure and set chassis routing-engine bios uninterrupt options are configured, there is no alternative recovery method available in case Junos OS fails to boot and the device might become unusable.
These types of features are about locking down the boot process for devices where you are concerned about physical access security to the device itself. This will prevent a malicous physical hands on actor from rebooting the device and taking over during the boot process to own the device.
As the doc notes doing so also means you won't have access to those boot time interuptions yourself should they become necessary in actual problem situations.
So this is a trade off between having a device fail in an unrecoverable way versus having the extra physical security.