Security bounties was announced in 2013 and initially rewards are in ounce of gold, exactly as you recalled.
Before finding the announcement thread, I checked Wayback machine but the first snapshot was in May 2021 and I actually thought you remembered it incorrectly.
May 2021 snapshot with rewards in bitcoin.
These bounties use a separate system of calculation, but are subject to the same conditions as above.
- 1 XAU: Find the email address of user DefaultTrust and explain in detail how you did it.
My guess, theymos changed it in May 2021 and made that snapshot on Wayback machine. It it was, that makes sense if theymos wanted to award as highest as possible in value for security bounty hunters. As since 2018, price of bitcoin is better than gold and has been continuing to rise more. Risk of dead for Bitcoin has been becoming smaller and smaller.