Where do you place the cat's litter box without creating any bad feng shui vibes? Good question. Your bathroom is not the best place because ideally, you want a spa feel in your bathroom, a kind of gentle retreat for your early mornings and late nights.
So where do you place the cat litter box, then, if you live in a small apartment?
I do not have cats so would not know all the aspects, or requirements for the cat's litter box. I can certainly say, though, that it is bad feng shui to have the cat's litter box in your kitchen or dining or in the bedroom.
Best I have seen is a litter box under a small table that somehow made it almost invisible to the eye (because of the low coffee table design), at the same time accessible for the cat. It was placed in a big family room close to the patio.
It might be harder to find a relatively good feng shui place for a cat's litter box in a small apartment versus a big house. You will have to experiment and see where it feels right/where it bothers you the less.
It might be that in a small apartment a cat's litter box ends up in the bathroom, just do the best with it. It will not create bad feng shui, as long as you do your work in terms of keeping the bathroom energy fresh.
You can leave the bathroom door slightly ajar, this will not be bad unless the bathroom is close to the main door or your bedroom.
Hope this helps you!