The product backlog should contain all future work to be done.
“A product backlog is a list of the new features, changes to existing features, bug fixes, infrastructure changes or other activities that a team may deliver in order to achieve a specific outcome.” — Scrum Alliance
This is a good definition because it points to a broad target for the product backlog. The backlog is not only for new features customers, the product manager, or the CEO, are clamoring for. It is really about all future work that will create value for customers.
It is important to remember the product backlog is not, strictly, a product management tool. This is because being a product owner is not the same as being a product manager. Your goals and audience are not the same. Surely, there is overlap, but it is not the same thing.
Find out more in What’s in a product backlog?