Link validity for a specific link is a function of the specific relationship and is not stored as part of the source or target of the link. For example if there is a validates requirement link from a test case to a requirement, the link validity status will not be represented in either the RDF representation of that test case or that requirement. Link validity data is stored in the JTS to which applications are registered. AFAIK, there is no public REST API to this. Link valdity is currently not covered by any OSLC specifications. There is a private API used by ELM applications to manage link validity data in JTS, such as get the link validity status resource for a particular source and target. The RDF vocabulary to represent link validity is described at https://jazz.net/wiki/bin/view/LinkedData/JazzLinkValidityVocabulary,