Leaving a cat's tail on your doorstep is disturbing and has no widely accepted symbolic meaning. It is likely a malicious or threatening gesture intended to cause fear.
What to Do
Stay safe, do not approach the object, and document the scene with photos. Report the incident to local police or animal control. Consider security measures if you feel threatened.
This is a serious matter; take steps to ensure your safety and the well-being of animals in your care.