Stream processing systems designed to process data streams in real-time must handle sensitive or personal data across multilayered systems (sensor, fog, and cloud layers), which raises privacy concerns as data may be subject to unauthorized access and attacks violating user privacy and facing regulations such as GDPR. This paper discusses privacy-preserving mechanisms (PPMs) proposed to protect user privacy in SPSs, noting that selecting and applying such PPMs is challenging since they must operate in real-time while tolerating little overhead, and that the multilayered nature of SPSs complicates privacy protection because each layer may confront different privacy threats requiring specific PPMs.