With IPv6 adoption accelerating over the past 6 years, from being a negligible fraction of the Internet (<1%) to recently topping 25%, moving IPv6 a full Internet Standard could not have come at a better time.
The Internet Standard designation represents the highest level of technical maturity and usefulness in the IETF standardization process. As the relative numbering of the RFC (RFC 8200) and STD (STD86) suggests, there are many protocols that make their way through the IETF standards process to be published as RFCs, but are not Internet Standards. The Internet Standard designation means those implementing and deploying a protocol can be assured it has undergone even more technical review by the IETF community than the typical RFC, and has benefitted from experience gained through running code and real-world experience. This is definitely true in the case of IPv6.
The IPv6 Maintenance started the process of elevating IPv6 to Internet Standard in July 2015. Early in this process, the WG decided to fold in several of the key updating RFCs into the new version of the IPv6 specification while maintaining interoperability with legacy implementations. The RFCs that have been subsumed are summarized in Appendix B. of STD86. Several of these updates were related to handling security issues (e.g. the deprecation of the type 0 Routing Header that could be used for traffic amplification attacks), fragment handling, extension header processing, and more precise specification of the usage of the flow label field. Also, as required by RFC 6410, all the verified errata filed against RFC2460 were evaluated and necessary changes were made to RFC8200 in order to fix these. One of the key changes made in this regard was to clarify the text regarding the handling of extension headers in intermediate nodes. This ended up being a very controversial topic with strongly held and justifiable opinions on both sides of the debate. After contentious working group and IETF-wide last calls, the current text in the RFC gained rough consensus as it was the safer choice that ensured interoperability.
In addition to the main protocol specification, some of the other key IPv6 specifications were also elevated to Internet Standard. Path MTU discovery for IPv6 (PMTUD) was also published alongside RFC8200/STD86 as RFC 8201/STD87. This version has been greatly revised based on deployment feedback acknowledging the relative fragility of ICMPv6 based mechanisms due to filtering on the Internet as well as incorporating extensive review and input from the IETF Transport Area. It also suggests the use of Packetization Layer Path MTU Discovery (PLPMTUD) [RFC 4821] in order to improve robustness in the presence of ICMPv6 filtering.
In addition to these two revised documents, two other key IPv6 related specifications, the ICMPv6 specification (RFC 4443/STD89) and the DNS Extensions for IPv6 (RFC 3596/STD88), were elevated to Internet Standard in place. i.e. their status was changed without any further text changes. These specifications have been widely implemented and deployed, and there were no errata filed on these that would cause any interoperability issues.
Moving these IPv6-related specifications to full Internet Standards matches the increasing level of IPv6 use around the Internet. The IETF community has steadily worked to ensure that the Internet is ready for the time when IPv6 is the dominant Internet Protocol. Work in a variety of IPv6-related IETF working groups, such as 6man and 6ops, continues, striving to make the Internet work better.