Application Programming Interfaces (APIs) are the prevalent interaction method for software modules, components, and systems. As systems and APIs evolve, an API element may be marked as deprecated, indicating that its use is disapproved or that the feature will be removed in an upcoming version. Consequently, deprecation is a means of communication between developers and, ideally, complemented by further documentation, including suggestions for the developers of the API's clients. API deprecation is a relatively young research area that recently gained traction among researchers. To identify the current state of research as well as to identify open research areas, a meta-study that assesses scientific studies is necessary. Therefore, this paper presents a systematic mapping study on API deprecation to classify the state of the art and identify gaps in the research field. We identified and mapped 36 primary studies into a classification scheme comprising general and API-specific categories. Furthermore, we identified five major gaps in previous research on API deprecation.