A lot of D&D settings have what you might call open historicity. (By 'historicity', I mean simply the quality of having a history.) They are thought, like our own world, to have an origin, and a timeline that begins some time in the distant past and may go on indefinitely into the future. The timeline, note, does not have to be detailed - it simply has to be the case that there is a sense that events in the world follow on from one another in a chain of causation. Faerun, Krynn, Eberron, etc., all tend to be like this, as do most 'fantasy heartbreaker' worlds and those in high fantasy novels (such as Westeros).
The classic world with closed historicity is Middle Earth. It has an origin and a timeline but also an end. Its historicity is not indefinite - there is a point at which Middle Earth as we know it ceases to exist. There are, I am sure, other example - Narnia? The Hyborian Age? Urth? Viriconium?
Then there are settings with absent historicity - which present themselves as having existed in the same form essentially forever, and generally without any indication that they have a history as such at all. Alice's Wonderland, Lovecraft's Dreamlands, Neverland, Oz and Fantastica are obvious examples.
It is not necessarily straightforward to deploy this taxonomy. Many settings which one would instinctively put in the 'open' camp turn out to be closed (Lyonesse, for example, or Zothique). And it is easy to fall into the trap of doing violence to a setting like Alice's Wonderland by falsely historicising it, as with the Tim Burton film and its sequels. There are also some interesting edge cases. Is China Mieville's Bas-Lag a setting of open, or absent, historicity? On the one hand, it gives the appearance of having historical depth. But on the other, it seems caught in a holding pattern - fast forward 10,000 years and once suspects that a lot of stuff will have happened, but that the same conditions will essentially prevail.