This feature had been heavily requested but it won't be added, it adds trust in a feature that wouldn't meet the objective people would want it to have. There will always be traces such a feature is either in use or was configured if they had file system access or other control. The device keeps the OS installed anyways so it can be recovered to a fresh install.

There's some justification here:

It wouldn't be good to assume a regime with power to locate you and kill you would be subverted by a simple trick. We are on many peoples' radars already as per leaked Cellebrite documentation describing their lack of GrapheneOS extraction capabilities. It wouldn't be far from the tree to assume the big guys describe GrapheneOS features to their customers and partners. A knowledgeable person also wouldn't trust a GrapheneOS user to comply with this feature.

You should never use GrapheneOS features to trick people, and to use the duress PIN that way isn't the way it's designed to be used. You can't really be sure that they will let you go if the trick worked or even if you complied either. If they can kill or torture someone without accountability then there'd be little disadvantage in keeping you or even killing you beyond them cleaning up the blood and guts. Duress is to protect data, not the device owner.