Signal can at least have timeouts for requiring re-auth (typically biometric). The isolation concept can indeed apply to more than just functional separation,

Signal can at least have timeouts for requiring re-auth (typically biometric). The isolation concept can indeed apply to more than just functional separation, including separation in time, or in search depth (number of records returned) or by classification. It would be quite simple -- require auth to scan backwards more than an hour. Or 'press and hold to mark message as sensitive|expiring'. And possibly require a passphrase to access those sensitive messages / threads. I've applied these techniques for database systems where NTK (need to know) rules apply, to force queries to be narrowly scoped. Likewise system backups don't all need to be online (or in the tape robot), most restores are from the most recent backup.