How to downgrade from Android 5 to 4.4.4 on an encrypted phone?
In L, take a backup of things you want to save (photos, videos, app data, etc)
Go download a stock rom for your device, such as this one
In TWRP, wipe data, system, cache and dalvik cache
On your computer use fastboot to flash the system.img 
Reboot
You should now boot into kitkat, without any encryption.
Restore any backups you might've had. Note that if you simply backed up the /data partition, that might cause problems when restored.

7b. If you did create a backup with twrp, you can still restore in a safe way
with titanium backup pro, assuming you root the device.