Why is the protocol called IPv6 and not IPv5, and when was it standardised?

The candidates were collectively called "IPng" (IP next Generation); the chosen one got the next free version number — 5 was already taken — so it became IPv6, standardised in 1998 (RFC 2460).

The naming story:

• Several competing proposals were developed under the umbrella name IPng — Internet Protocol next Generation
• The IETF picked one and gave it the next available version number in the IP header's 4-bit version field
• Version 5 was already reserved (for an experimental streaming protocol, ST), so the winner became IPv6
• Result: IPv6, RFC 2460 (1998) — later updated/obsoleted by RFC 8200 (2017)

Tip: The version number lives in the first 4 bits of every IP packet — 4 for IPv4, 6 for IPv6 — which is exactly the first thing a packet analyser reads to tell them apart.