The term "OSX 21" refers to the macOS operating system version, which is compatible with the cracked plugins. The "free" part of the equation is, of course, a major draw for producers and engineers on a tight budget.
Regarding the cracked version for OSX, I must emphasize that using cracked software is not recommended. Cracked software can pose security risks, may not function as intended, and can also harm your computer. Additionally, it's essential to respect the intellectual property rights of software developers.