No, a kitchen sink does not necessarily need to be on an outside wall. The location of the sink in a kitchen is determined by the plumbing and the layout of the space. While it is common to have a kitchen sink placed along an outside wall for easier access to plumbing lines and drainage, it is not a strict requirement.
In many homes, especially in older constructions, you may find kitchen sinks positioned on interior walls or on kitchen islands. Modern plumbing techniques and technologies allow for more flexibility in designing kitchen layouts. As long as the sink has access to proper plumbing connections and a drainage system, it can be placed in various locations within the kitchen.
When renovating or designing a kitchen, consider factors like the existing plumbing layout, cost of rerouting plumbing lines, and practicality in determining the best location for your kitchen sink. It's essential to consult with a professional plumber or a kitchen designer to ensure the sink's placement aligns with your needs and meets local building codes.