fxnn
If you set up a guest profile under Settings> System> Multiple users, there is a "Delete guest activity" button.
The hint below reads as if all guest profile data is removed when activated: "Delete all applications and data when guest profile is closed".
And from the GrapheneOS FAQ (which you thankfully already linked to - just quoting it again for ease of use):
https://grapheneos.org/faq#encryption
Sensitive data is stored in user profiles. The user profiles each have their own randomly generated hard disk encryption key, which is used for encryption.
and
File system-based encryption is designed to allow files to be deleted without having the keys for their data and file names, which allows the owner profile to delete other profiles without them being active.
and from this post (which talks about the pros and cons of user profiles):
https://discuss.grapheneos.org/d/4867-secondary-user-profile-as-a-main-one-advantages-and-disadvantages
To improve forensics, you can delete profiles with a keystroke, which deletes the encryption keys and leaves minimal to no artifacts since all profiles are encrypted.
This reads to me (someone please correct me if I am wrong) as if access to the data is not possible and purging a profile leaves at most minimal traces in the form of artifacts.
23Sha-ger Very cool idea to use the guest profile like this, by the way, I like it.