Replying here so I can do two in one:

GrapheneOS does not have such a feature yet but a PN/password that erases the device is being worked on. Oftentimes people use an app like Wasted, Duress or similar which allows a trigger to erase the device via device admin. We never recommended these because all these duress features that people sell or make apps for in this method can be easily bypassed by holding the Volume Down button when the trigger happens to boot to Fastboot and cancel it. We have a video by a forensics company (MSAB) instructing how to perform this bypass, it has been common knowledge for ages.

This is why this work has took longer than people think, because the way GrapheneOS does it should not be cancelled or bypassed. It has to be new.

The automatic reboot covers a threat actor seeking to attack a device by taking advantage of it being in an AFU state. This is not the only protection we have on offer. The very new USB port controls are another. The hardware-backed rate limiting via the Titan M2 is a part of the benefits of the hardware we support. The longer passwords also means having ones so long you cannot brute force (providing the entropy is great!)

Regardless of autoreboot, your phone won't get any updates if seized. They could wait and see if they can exploit in the future... but if a phone is in BFU their work becomes harder. They'd need to find a way to exploit the tamper-resistant secure element to even think about getting through brute force protections most likely.