In some newer systems (the Nintendo Switch) there are hardware fuses they blow with each update. Their software won't work if the appropriate fuse is already blown.

The PS4 firmware is encrypted with a per-console key embedded in the CPU, so to downgrade you would need a copy of the old firmware taken from the same console.

Technically you can downgrade to exploitable firmware but only if the system had exploitable firmware in the first place, and had its firmware backed up, before being updated.

Don't know about the ps3 but the xbox one has efuses that get blown with most firmware update and prevent downgrading in any form, don't know whether any workaround has been found yet