CoAP is specified in a Standards-Track RFC. RFCs that serve as Internet Standards Documents are generated by the IETF based on an extensive technical review and quality control process. These are meant to last.
The stable reference.
Approved 2013-07-11 as draft-ietf-core-coap-18, it has been published as RFC 7252 on 2014-06-26 (the delay having been caused by waiting for some related security specifications to finish their publication process).
CoAP has been designed to be extensible. Several registries are defined to enable evolution of the protocol over time. This includes response codes (such as 4.04 "Not found") and content-format values to indicate media types (such as 50 for application/json). These are collected in an IANA registry to ensure specifications are available and that they don't stumble over each others code points.
Several other specifications work together to support using CoAP.
RFC 6690 defines the CoAP link-format, a simple format for exposing the resources offered by a CoAP server as the basis for a resource directory.
Employing the basic multicast functionality defined in RFC 7252, RFC 7390 provides additional information and protocol flows for how to use CoAP for group communication.
CoAP is being picked up by other organizations as a normative reference for their standards.
On top of CoAP, the Open Mobile Alliance (OMA) has defined “Lightweight M2M” as a simple, low-cost remote management and service enablement mechanism.
Besides a considerable number of commercial implementations, open-source implementations of the LWM2M standard are available for the client and server sides. Some basic testing support is also available on-line at coap.me.