General:
* Extinct: This applies when there are no surviving members of a species or group.
* Unsubstantiated: This means something is not supported by evidence, including living proof.
* Unconfirmed: Similar to "unsubstantiated," this indicates a lack of verification, including from living sources.
* Unproven: This describes something that has not been established as true, which might be due to a lack of living witnesses.
More specific:
* Mythical: Used for creatures or events that are believed to be fictional.
* Legendary: Similar to "mythical," but might imply some possibility of truth, even if not confirmed.
* Historical: If the subject is a past event or person, this might imply a lack of surviving eyewitnesses.
Phrases:
* No living soul can attest: A more direct way to say "no living proof."
* No one alive remembers: This emphasizes the lack of witnesses.
The best choice depends on the specific situation you're describing.