Do you have a reference for when the Internet as a whole was supposed to cut over to IPv6 only?

There are plenty of organizations that have transition mandates but those are self imposed and would easily be delayed for other monetary business objectives. The Internet as we know it was only created/available in 1993[1] and while some RIR's have exhausted their available IPv4 space it doesn't mean IPv4 addresses are not available.

[1] https://www.npr.org/2023/04/30/1172276538/world-wide-web-int...