Sunsetting, in a business context, is intentionally phasing something out or terminating it.
The term is often used in reference to brands, partnerships, agreements, business policies and hardware and software products, among other possibilities. Sunsetting something may be part of the original plan. In a legal context, for example, a sunset provision is a specification that legislation will no longer be in effect beyond a certain date or after a particular event has transpired, unless an extension is mandated.
Products and services are often sunsetted when they are no longer sufficiently profitable or when a company decides to change its focus. Software vendors usually sunset older versions of their products when one or more newer version is available. In that context, sunsetting usually means that the product is no longer supported.