Technical Debt and Done in the Scrum Guide
Although the Scrum Guide is quite clear on 'Done', nowhere is technical debt mentioned explicitly. I read 'Essential Scrum' by Ken Rubin in which he defines different types of technical debt, some of which are avoidable, as you happen on them during development and are thus prevented by adhering to the definition of Done. However, some are unavoidable - for example the passage of time makes some code obsolete. If we don't happen upon them during development but the code did adhere to the DoD when it was created, what do we call this?
It makes sense that the Scrum Guide doesn't talk about technical debt, since it's not applicable to all products or services that may be designed and delivered with Scrum. It's a concept that originated in software engineering, but may be applicable to other domains as well, but is not a concept that exists in all domains in which Scrum may be used.
That said, why is the term "technical debt" not appropriate for cases where work that once met the Definition of Done no longer meets that definition?
I would point out that, in the Scrum framework, any work that needs to be done to improve the product is a Product Backlog Item. Talking about specific types of work, such as technical debt, may be useful for the team, but it's not part of the Scrum framework. New functionality, defect fixes, technical debt, and any other types of product development are all treated the same by the framework.
Generally speaking you might still be Done and build up technical debt in the Product, since it can be incurred unconsciously. The Sprint Retrospective presents a formal opportunity to improve the Definition of Done, so once this debt is surfaced it can then be managed.
Since technical debt is not explicitly mentioned in the Scrum Guide there is no hard and fast usage of the term. When "undone work" is not yet accounted for on the Product Backlog, you might prefer just to describe it that way, or perhaps as a technical liability or as an unquantified and unmanaged loss.