If you are a developer wanting to test app compatibility, a GSI is the cleanest way to run legacy software on modern hardware (assuming your device supports treble and you can unlock the bootloader).
If you have an older tablet or phone that struggles with modern bloatware, a KitKat installation can breathe new life into the hardware. android kitkat os download
The most popular way to "download" KitKat today is through Custom ROMs. Developers have stripped down the OS to work on specific older devices. If you are a developer wanting to test
If you just want to relive the interface or test an app without risking a phone brick, use an emulator. use an emulator.