Recently, mission-critical Industrial Internet of Things (IIoT) applications such as system automation, predictive maintenance, and anomaly detection have come into the spotlight of Industry 4.0 thanks to the promised benefits. The IEEE 802.15.4 Time-Slotted Channel Hopping (TSCH) mode, along with the IPv6 over TSCH (6TiSCH) initiative, are two key standards to accommodate the diverse traffic patterns, reliability, latency, and power efficiency needs of such IIoT applications. To manage the allocation of communication resources in TSCH networks, a Scheduling Function (SF) is implemented. Even though scheduling in the IIoT has been the subject of numerous reviews, the potential of taking traffic-awareness into account has not been fully investigated. Motivated by these facts, we classify and analyze, in this systematic mapping review, prominent SFs dealing with traffic-awareness in TSCH networks published between 2012 and 2022. As a result, we provide a multi-dimensional map to identify the current trends in traffic-aware TSCH scheduling and help assess how far a given proposal is supported or contradicted by the empirical evidence in the field. Consequently, we discuss some open challenges that require community attention and point out potential future research directions regarding the design, implementation, and evaluation of new traffic-aware SFs.