Are accomplished requirements still part of the Product Backlog?

Question

The offical Scrum Guide states that

The Product Backlog is an ordered list of everything that might be needed in the product and is the single source of requirements for any changes to be made to the product.

Does this mean that the Product Backlog also contains requirements that have already been successfully implemented (according to their Definition of Done)? Or does it only contain requirements that have yet to be actualized?

Answer 1

A backlog, by definition, is "an accumulation or buildup, especially of unfilled orders or unfinished work". A Product Backlog is only the work that is known to be remaining on the product, just like the Sprint Backlog is only the work that is remaining to be completed in the Sprint.

The Scrum Guide concurs for both the Product Backlog:

The Product Backlog lists all features, functions, requirements, enhancements, and fixes that constitute the changes to be made to the product in future releases.

and for the Sprint Backlog, too:

As new work is required, the Development Team adds it to the Sprint Backlog. As work is performed or completed, the estimated remaining work is updated. When elements of the plan are deemed unnecessary, they are removed. ... The Sprint Backlog is a highly visible, real-time picture of the work that the Development Team plans to accomplish during the Sprint....

Answer 2

If you read a bit farther in the Scrum Guide page you linked, it also says:

The Product Backlog lists all features, functions, requirements, enhancements, and fixes that constitute the changes to be made to the product in future releases.