There are two ways of answering this question.
First, at the programming level, the item is typically an object, and the list is an object that can contain other objects. The ability to add and remove objects is usually a property of the container. There is no special adjective for the object.
At the higher semantic level it’s different. The item is represented by the object, but it also corresponds to something in the external world. It can have significant properties of its own. For example, the item can be addable to a list, or removable from a list, and the list itself contains items that are subject to some further action, e.g. bought, cooked, painted on a boat, etc. However, at this point you need an adjective that works with in the external world with both the item and the collection, and without more information it’s hard to give a good answer.
That said, here are some general adjectives that apply to items that can be added and removed from collections: