Newer versions of Kontakt require modern operating systems (Windows 10/11 or macOS 12+). For those running legacy "frozen" studio computers on Windows 7 or older OSX versions, 5.3.0 is a stable peak.
Native Instruments Kontakt 5 v5.3.0 Unlocked: A Legacy Perspective
It enabled the loading of non-encoded libraries that usually required a "Player" license.