Release 8.5.171.0 is part of the 8.5 MR (Maintenance Release) train. It is not a "golden" release in the strictest sense, but it served as a highly anticipated update that consolidated fixes from previous, less stable iterations. Released in a window where the industry was shifting toward 802.11ax (Wi-Fi 6), this version provided a necessary stability patch for organizations not yet ready to rip-and-replace their hardware for the newest generation of controllers.
However, it is crucial to note the divergence from the 8 cisco 8.5.171.0
For newer hardware like the 5520 or 8540, Cisco recommends migrating to the 8.10 train for continued security support. Release 8