We don't have this feature as a priority because it would be detectable with forensic analysis. People would just trust the feature too much thinking its existence would be hidden when it would only trick a bystander. GrapheneOS is well known, they'd just treat any device installed as likely to have a hidden profile.

We suggest keeping things in a separate profile and deleting it when things get heated. No data is worth keeping if you're being targeted over it. If you were someone like a whistleblower you'd ideally provide copies to others or make encrypted backups to a trusted cloud they aren't aware of should the device be taken.