There may have been plans to include minable iron ore deposits in the game overworld at one point in development. It is possible those might make it into the game some day, but I wouldn't count on it. Muddy scrap piles can be found even outside of crypts with the use of the Wishbone. Their presence makes iron deposits redundant.

If I were to speculate, I might think that the player could make steel using iron and some other resource. Rather than send the player back to the swamp, a new biome may have iron deposits with greater ore return to supplement a poor exchange rate (e.g. 2 copper + 1 tin = 1 bronze).

I believe there are iron ore deposits in the swamp, but finding them without an item that helps you find silver is pretty hard.

The iron ore is in the code, but not in the game, same as the blue mushrooms. Never been confirmed if it was something that didn’t get used in favor of iron scrap, or if it’s something for later biomes.

I'm guessing that the devs created iron ore/ore deposits early on with the assumption that all the common metals would be discoverable in their rawest form. Either that or they plan to add more ways to discover iron at a later point, that maybe wouldn't require returning to the swamps. Right now neither iron ore or iron ore deposits will spawn naturally:
https://valheim.fandom.com/wiki/Iron_ore
https://valheim.fandom.com/wiki/Iron_vein

Originally posted by GrouchyMike:

I believe there are iron ore deposits in the swamp, but finding them without an item that helps you find silver is pretty hard.

You are mostly right except it is muddy scrap piles, not iron ore deposits that can be found hidden in the swamp.

Another way of also finding scrap iron without crypts is by killing oozers (big brown blobs, not the little green blobs), but I wouldn't blame anyone for forgetting or not knowing this because I assume most would not consider them worth the trouble of killing.