Piece of garbage software, it encrypts your device and has a button for wiping the device clean. This has been used by someone at a border inspection... Which amounted to destruction of evidence/evidence tampering. All of this is apparently by design, not some obscure misuse of the technology.
Even if you believe that customs is immoral or whatever, it is a strategic blunder to use such a button, if your device is properly encrypted it's virtually inaccessible, the only thing wiping the device does is add a criminal charge.
If you wanna larp, at least larp as someone smart, not someone that fabricates a criminal charge out of a routine inspection.
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Android and IOS encrypt your device. Most modern phones do, that is not specific to GrapheneOS and is an important, generic part of data safety that has been used for decades.
GrapheneOS offers a duress PIN/password, not button, that is solely up to the user to use as they see fit. The threat model for its use is on the user to determine, and the example you are referring to was a judgement that person made. That decision is independent of GrapheneOS.
Whether or not it was a blunder can only be known by the person who used it. Maybe it was a miscalculation, or maybe they were trying to hide something of importance, like protected contacts in an authoritarian country. We cannot know.
Duress PIN is for when the consequences of having the data are worse than erasing the data. This is very important for journalists or citizens of an authoritarian government. The judgements these people make are not a reflection of GrapheneOS.