Iodine is also found in seafood and dairy and eggs, which may have been less common in the American diet a century ago, especially among the poor, but are widespread today. Americans no longer need iodized salt. (Not sure about vegans.)
It's definitely in seafood. Dairy and eggs depend on what the cows and chickens have been fed. In regions with low soil iodine content, you can expect the eggs and milk to also be low in iodine.
Vegetables and grains also have a significant amount of iodine in that sort of soil (it's where the cows and chickens get it).
Vegans should already be extremely watchful of their nutrition, as there's for example B12 in a lot of animal products, where a B12 deficiency means you are getting nerve damage.
