This research systematically maps literature concerning the application of unsupervised machine learning approaches to test suite reduction (TSR), a critical process for optimizing software testing efficiency. The study, which reviewed 34 papers published between 2013 and 2023, identifies common algorithms and evaluation metrics in this field. It highlights K-Means clustering as the most frequently used algorithm and coverage metrics as the primary means of assessing effectiveness. The findings also point to a significant gap in the literature regarding scalability considerations and a general lack of shared research artifacts. Despite these challenges, the research underscores the broad applicability of unsupervised learning for TSR across various software domains, from web-based applications to embedded systems.