I see two broad options, one of which is a bit of a frame-change.
First, the frame change: most undead - especially low-level undead like zombies and skeletons - were mindless in previous editions of D&D and aren't terribly bright in 5E. It shouldn't be too hard for some passably persuasive denizens to convince the undead hordes to follow their commands without magic. Further, zombies and skeletons are frequently portrayed as basically milling about doing nothing in particular if there's nothing better to do; it shouldn't be too hard for the denizens to have corralled the undead into holding areas for later use (it's not like they need food or exercise, after all).
Option 1b, I guess, would be that there's no particular reason for the undead creatures to stop doing whatever it was that they were doing when they're no longer controlled, especially if what they were doing is a mindless, repetitive task ("keep turning this wheel" or "walk along this round pathway (ie., patrol)").
If that's not quite sufficient, the denizens could be keeping a handful of key undead creatures under their control, and trusting that the rest of them will just kinda follow along.
PCs could, in principle, do the same thing: control or convince (or just lure) the undead to where you need them to be, then let them hang out there 'til they're needed. If necessary, keep a small number under control for specific purposes.
The second option would be to adjust (slightly) the Hallow spell, and "Un-Hallow" the area (which may indicate that a more powerful creature is behind this, and the adventure is prelude). As written, Hallow appears designed for Good PCs to use; I don't see why its mirror shouldn't exist, which would allow for necromancers to exert some control over more undead than normal (possibly only just "some" control, though: perhaps it's just enough that the undead see the other denizens as "friendly" as opposed to "lunch").
Again, in principle a PC could do this, if they could find a deity willing to allow it.
Master Chronology
Based on the link provided by Mark Horsfall, and the information Paul Westermeyer has compiled and compared in the linked article, the following information has been derived:
Sources
The three primary campaign settings for D&D are Greyhawk (GH), Forgotten Realms (FR), and Dragonlance (DL). These three have more material published than the other settings, and include the majority of the cross-time references. (Which other settings are: Spelljammer (SJ), Maztica (MZ, part of FR), Al-Qadim (AQ, Also part of FR), Mystara (MY), Ravenloft (RL), Planescape (PS), Birthright (BR), Athas (AT), Red Steel (RS), Odyssey (OD), Lankhmar (LM), Eberron (EB) and Conan (CO).)
There are four additional primary sources for chronological cross-time points: Spelljammer, Planescape, Ravenloft, and Dragon Magazine (DM) articles written by Ed Greenwood himself.
Please note that there are some contradictions in sources, so precedence is given to authors with more published works and to sources based on Spelljammer (as it is specifically designed to cross-bridge the settings) as well as the Big 3 (FR, GH, and DL).
FR/DL cross-time link
Based on comparative information from SJ, FR, DL, RL, and PS the best cross-time linkage is:
1361 DR <-> 358 AC
The RL data contradicts itself, weakening it as a source, and the PS data fails to trump the SJ data.
GR/FR cross-time link
Based on data from FR/RL/GH and DM sources the best cross-time linkage is:
1361 DR <-> 581 CY
Note that the DM articles were written by Ed Greenwood about the wizards Elminster, Mordenkainen, and Dalamar meeting in his home on Earth to discuss the magical affairs of Faerun, Oerth, and Ansalon with reference to various famous wars and other events; and as such provide a strong primary source for cross-time linkages. As such, the dates of these articles can be used to provide an Earth-time linkage as well, should such be desired.
Other cross-time links
Linkages to other settings frequently fall back on the publication date of the earliest release of said setting, combined with in-setting references, or lack thereof, to determine dates. For detailed reasonings, check the link if available - and I have saved a copy in case the link goes down.
Calendars
After all is compared, the following calendars can be synced, with confidence:
- Forgotten Realms: Dale Reckoning (DR), Shou Lung (SL)
- Greyhawk: Olven Calendar (OC), Common Year (CY)
- Dragon Lance: Ansalonian Calendar "Alt Cataclius" (AC)
- Spelljammer: Time of Unity (TU), Anno Vulkarus/Promo Novo (AV), Astromundi Chronos (AstroC)
5043 OC = 581 CY = 358 AC = 1361 DR = 2611 SL = 465 AV = 856 TU = 9/1299 AstroC
This corresponds roughly to the 1990's on our Earth, for those interested.
Best Answer
In general, you're going to be hard pressed to find a very large area where magic does not function or is entirely forbidden, for the simple fact that D&D's adventurers have always been highly magical and dependent on magic. A place where none of your cool stuff works anymore or where several character classes become almost entirely useless is not likely to be a fun place to set adventures, so they aren't going to take up much space in a published setting; places where the rules of magic are just a bit different are more common.
If you're happy to operate at a very small scale, though, there's a couple of spots in the Forgotten Realms which you might look into. FR is normally an extremely high-magic setting and the dead-magic zones which do exist are not large enough to fit an entire kingdom in by any stretch, but there are:
The City of Tantras
During the events of the Time of Troubles, the gods Torm and Bane fought each other in the city, the end result of which being that the gods slew each other and the battle created a dead magic zone covering the entire northern quarter of the city ("a zone of some 400 acres", apparently).
The rest of the city still has magic, and the dead magic zone itself isn't a pleasant place to go as it became widely used by criminals hiding from magical detection.
The Valley/Village of Dead Tree Hollow
The valley now known as Dead Tree Hollow was turned into a dead magic zone a few hundred years ago, after a vengeful mage furious at being rejected instead decided to murder the unfortunate lady and everyone around her.
The valley is small, but large enough that it contains a small village and surrounding lands, so it could be suitable as a no-magic location for small-scale adventuring. Note that even in this case it's not as if magic will be unknown to the villagers that live there - they would not have to travel very far to find places full of wizards and clerics.