The Past Is a Verb

Buried among the inequality of its typically-implied feudal setting, D&D has an implicit (and oft-ignored) transhumanist streak.  High-level D&D magic breaks out of the utilitarian doldrums and goes fully post-singularity, creating the wretched cyberpunk dystopia of your nightmares.  (And that ignores the possibility that your setting is literally post-singularity — all hypertech virtual GUI interfaces and half-mad shattered AIs as gods.  Jeff Rients' World of Cinder operated under this paradigm, as does Anomalous Subsurface Environment, just to give a couple of examples.)

Perhaps more importantly, there exists the very real possibility that the world is haunted by the ghosts of its past realms.

The caveat that needs to be acknowledged: along with Star Trek, D&D isn't great at fully teasing out the inevitable consequences of its implied setting.  (Two examples of Star Trek: they discover easy time travel that they rarely use and they introduce warp drive as causing an ecological disaster then completely abandon that subplot.)  I'm not entirely certain it should be — we're just trying to run a game, not fully indulge in masturbatory world-building — but it's a fun little exercise to contemplate how a world would develop if someone can alter reality because they went to grad school, or because they have fire demons in their blood, or whatever.

So D&D is always figuratively about some mythic past as per Tolkien or Chaucer: "In th' olde dayes of the kyng arthour, / Of which that britons speken greet honour, / Al was this land fulfild of fayerye. / The elf-queene, with hir joly compaignye, / Daunced ful ofte in many a grene mede. / This was the olde opinion, as I rede; / I speke of manye hundred yeres ago. / But now kan no man se none elves mo."  Adventurers interact with their environment in geologic time, encountering the earlier epochs of the realm's history as they delve deeper into the earth.

But there are a couple of significantly more visceral methods of interaction with the past, and they tend to be frequently neglected.

A Boot Stamping on a Human Face — Forever

It varies, of course, but most D&D settings assume some magic, even if it's past its heyday (again as per Chaucer and Tolkien).  Unless you're in the lowest of low magic worlds, chances are good there's at least one vaguely approachable high-level wizard running around.

Although perhaps there are good reasons to avoid the wizards in your setting.

If said wizard is vaguely approachable, then it makes perfect sense that she would be making fat bank casting high-level spells (or scribing high-level scrolls, or manufacturing high-level artifacts) for people who can afford them.  Once you get over the novelty of bossing people around with charm and dominate, or just manufacturing items on demand with fabricate like your own 3D printer, any aristocrat worth their salt is going to want to hang onto their wealth, preferably forever.  So you get into high-octane, death-defying spells like magic jar (researched or imbued in a scroll so that others can use it, of course) and clone.

(While we're on the subject, this is a pretty great magic jar spell analysis.)

At this point, you now have a gentry with the ability to live forever, so long as they can keep pumping money into the system.  It's Courtney Campbell's lich from his tricks supplement, representing "our fear of ancient rulers imposing their unending rule upon us," but describing mortal, magically-augmented people rather than the undead.

But it's basically the same effect: your ancestors bossing you around forever.  Entropy never wipes the slate clean; the King must truly have divine authority if the King rules eternally.  Altered Carbon and Tini Howard's sadly-unpublished Chamber Sounds both delve into this concept — the bastards who get there first gatekeep the rest of the schmoes trying to defy death.

And that's to say nothing of a potential hegemony of wizards capable of doing these things, although that concept is more frequently examined in D&D spaces (potentially including this very blog!) than the idea that the rich use their money to also benefit from said magic.

You can also get into a bunch of interesting permutations, like the magic-users capable of crafting soul jars or clones also forming a protection racket whereby they'll keep these important relics safe for a nominal fee.  (And they'll blackmail you with the knowledge that you won't regenerate after your next death if you don't play ball.)

In Lordran, the Flow of Time Is Distorted

If you don't want to get too heavily into the transhumanist implications of potent D&D spells, then there's a more apparent aspect of D&D that probably merits examination.

Several spells are capable or returning the long-dead or otherwise preserved back to full life.  There's a good example of some manner of greater restoration bringing a new player into the party (after prolonged existence as a statue) in this video:

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A similar occurrence happens in Dark Souls II:

Between restoring petrified characters, or using resurrection or true resurrection to restore the long-dead, it's entirely possible to have characters from historical epochs intersect with the present day.  And that's without getting into bits of strangeness like long-lived elves, speak with dead, contact other plane, or whatever other magic one can find or devise.

Any historical fact-finding mission or archaeological dig can easily turn into a diplomatic mission when it turns out your primary sources can talk.  Doubly weird because then they ostensibly integrate into society and inform that society — what happens when your ancestor whom you hit with a stone to flesh spell in the Temple of the Medusa tries to rejoin society a couple of generations after his contemporaries lived and died?

That's not, strictly speaking, a common occurrence, but it no doubt happens a non-zero number of times in any given fantasy society.

Now What?

Well, whatever you want, of course.  If you want to take it to its logical conclusion, you have a cadre of elite wizards and super-rich nobles who can live and plot and scheme forever, viewing themselves as gods and the common folk as pawns.  (Or maybe just a cabal of immortal and secretive sorcerer-kings moving behind the scenes.  Or maybe a royal dynasty and their pet wizard.  Whatever floats your boat.)  There are a bunch of ways to play that scenario, with the caveat that the balance of power is likely to be the sort of precarious situation that player characters can inadvertently tip one way or the other — destroy a clone vat or cast a dispel magic in the right spot and suddenly everything is in chaos!

With regard to the other point, it's probably not a terribly common occurrence (unless you have a lot of basilisks in your world or perhaps just one mad resurrectionist), but it is notable that the past sometimes has a more direct voice in the present.  Plus that lets you link in all manner of weird, ancient plots into your games — imagine a cult that leaves a handful of sleeper agents, bound in stone, and when some do-gooder adventuring party restores them to life, they can continue wherever they left off.  That sort of thing.

If nothing else, it's an additional source of clues, characters, and plot hooks, to be sure.