Your cat should have access to a well maintained litter box even if you have a pet door.
With a conveniently placed PlexiDor, your cat may choose to do their business outside, but it’s still a good idea to offer a litter box. You may for instance need to lock the cat door from time to time, and your cat may still need to use the bathroom. Always having access to a box minimizes the risk of accidents – your cat knows where to go.
If the weather outside is really bad, your cat might not want to leave the house. Having a box helps battle the temptation of using a soft rug. And, some cats prefer to use a box.
As an added bonus, your cat being used to a litter box makes it easier to adapt to travel or being with a cat sitter.
Make sure the box is big enough, placed in a safe, calm, and quiet area, has enough litter, and is cleaned on a regular basis.