If a home has multiple stories, an elevator that provides access to every floor is necessary for true universal design. It’s the only option that will accommodate every homeowner, guest, or tenant.
There’s a popular opinion that it’s adequate to stack closets or add other features in floor plans that can support the future installation of an elevator, though this caters to a “I don’t really need it” mentality. While the intention is good, accessibility will be limited, which is not what universal design is about.