I want to write a campaign where magic is totally absent using an existing setting—like an existing area where magic is strictly forbidden by authorities or an existing dead magic zone.

For the purposes of this campaign I don't want to create my own world or my own setting in an existing world.

If possible, I would like an area that is as medievally-realistic as possible. Like, not the city of Sigil or a desert area with dead magic or a no-magic plane.

The ultimate goal is to make a campaign that would seem medievally-realistic hence the need to have no apparent magic possible.

Are there areas in Forgotten Realms, Greyhawk, Dragonlance or other wizard official worlds that would work for this? What areas are there where magic is forbidden or "dead"?
Im open to areas in Pathfinder as well, but don't want to go too far off official paizo or official wizard of the coast.

If the area has no non-humans it's best, but I can do with them as well.

Forbidden is cool too, because if any magic is considered heresy, it still feels kind of historical.

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.

D&D doesn't have to include magic

The DMG mentions includes a "Magic in Your World" section (pages 23-24) that describe that magic may not be as common is some worlds as others. In particular, the "Restriction on Magic" section on page 24 says:

Some civilized areas might restrict or prohibit the use of magic. Spellcasting might be forbidden without a license or official permission. In such a place, magic items and continual magical effects are rare, protections against magic being the exception.

There's also the "Flavors of Fantasy" section (pages 38-41), showing that different flavours or genres of fantasy can lean more towards horror, mystery, heroic adventures, etc. It's entirely possible to simply have a world without magic that's more like a more realistic medieval world (but with monsters in it, still).

Since you mentioned not wanting to come up with your own universe (yet), you could simply use something like the Forgotten Realms and tweak it to downplay or remove any references to magic, but otherwise use the world as-is where it doesn't contradict your "no magic" requirement.

Obviously the classes that your players have access to will reflect that

Assuming you don't want any magic at all, then there are certain classes I assume won't be available to your players, such as Bards, Clerics, Wizards, etc. but also certain archetypes, such as Eldritch Knights, Arcane Tricksters, etc. There are a few edge cases that you might want to think about, such as Monks; is their "Ki" magical? Or are you willing to overlook that one? I imagine the Way of the Four Elements archetype would probably be restricted, though...