You can put a microwave in a landfill although it's best to not just throw the microwave away into your garbage.
Microwaves can contain hazardous waste and also contain some valuable metals including copper and the metal itself from the microwave can also be scrapped and recycled.
It's best to place the microwave out on the curb for some scrappers to pick up to recycle it instead of sending the microwave to the landfill.
Or if your area has any local recycling centers you can drop the microwave off there as well.
I scrap out my old microwaves and other microwaves that I find on the curb and eventually it adds up to some money so it's worth it for a scrapper to come by and pick the old microwave and other appliances up.
Many microwaves and other appliances eventually do end up at the landfill in some areas but in other areas they are often recycled.
Putting a microwave in the landfill is not good for the environment so it's best to keep the microwave out of the garbage and let a scrapper recycle it instead.