We have recently upgraded our Clarity instance from 15.9 to 16.0.2. We are facing some issues with the APIs.
Release : 16.0.2
To return roadmap items you should make a call with something like this.
http://your_server:port/ppm/rest/v1/roadmaps/5000001/roadmapItems
Also please referrence the swagger for any additional information and most accurate information on the given version.
http://your_server:port/niku/rest/describe/index.html#!/Roadmap/CaPpmGETroadmapItemsEntity