Search XDA Developers forum for “Magisk v20.4 download.” Trusted forum moderators often host mirrors.
Because these are legacy versions, you should avoid untrusted "one-click" sites and use reputable archives: download magisk manager for android 4.4.2
The industry standard for Android 4.4. It is more stable for KitKat than Magisk. Search XDA Developers forum for “Magisk v20
After extensive testing across multiple KitKat ROMs (CM11, SlimKat, Stock TouchWiz), the final stable release that supports Android 4.4.2 is: download magisk manager for android 4.4.2