Correct, no custom firmware, and it appears no downgrading your firmware to an older version without this restriction.

It's a trap-door upgrade.

Probably means that there is now a restriction on the firmware upload page of the router, so that it won't accept any arbitrary image, most likely implemented via signing.

Doesn't necessarily mean you can't load your own firmware in, but it would raise the difficulty of doing so. Finding a serial console on the board, vulnerabilities in the software, things like that - more akin to jailbreaking/rooting your phone.