The REST API is still part of Nexus of course. Since the API sometimes changes, you should look it up within your own Nexus instance and version! You can find it in the Plugin Console with the Restlet 1x Plugin.
More details are here: http://books.sonatype.com/nexus-book/reference/confignx-sect-plugins.html
If you want to have a local copy and e.g. your admin does not give you access, you can just start up a fresh install of Nexus on your machine locally. You will be admin on that fresh install and be able to see the docs. Also keep in mind you pretty much need the REST API only if you are developing some sort of integration. If you are doing that you should test against a local Nexus instance anyway... so you will have the docs right there.
Also if you know the entry point to the docs you can access them directly. E.g. https://oss.sonatype.org/nexus-restlet1x-plugin/default/docs/index.html