Kontakt 5.6.6 runs natively on older Intel-based Macs. It works seamlessly on OS X El Capitan, macOS Sierra, and macOS High Sierra.

Kontakt 5 will not run natively on M1/M2/M3 chips; it must be opened using Rosetta mode within your DAW (Digital Audio Workstation). Installation and Activation Can I update Kontakt 5.6.6 on Mac OS 10.13? - Community

Furthermore, version 5.6.6 holds a unique place in the alternative software community, a reality that cannot be ignored when discussing its ubiquity. On macOS, the security protocols implemented in later OS versions (such as the mandatory Notarization requirements in macOS Mojave and Catalina) made running unauthorized or "cracked" software increasingly difficult. Consequently, a vast number of users—particularly hobbyists and those in regions with limited access to paid software—froze their development cycle at Kontakt 5.6.6. This specific build became legendary in these circles because it was widely perceived as the most stable, pre-security-lockdown version available. It created a schism in the user base: those who moved forward with legitimate updates and Kontakt 6, and those who remained anchored to 5.6.6 to maintain access to their existing library ecosystem on older Mac hardware.