It seems that a demo is indeed coming imminently. As spotted by Wario64 on X/Twitter, a trailer for a Final Fantasy VII Rebirth demo can currently be seen on the Explore tab on the US PSN. This trailer appears on the US PSN for now, but has not appeared in other territories at this time. This trailer appears on the US PSN for now, but has not appeared in other territories at this time.

It would appear the trailer has been published early, as it says "demo available now." We can probably assume that was meant to air during the State of Play on February 6.