Can unprotected buried metals be harmful to soil?

Iron (steel) and aluminum are not particularly harmful.  They both will oxidize over time and turn into common minerals that occur naturally.

The harm comes from not recycling the metals, which causes more ore to be dug up somewhere else, then processed, to replace the buried items.

All this is not necessarily true for other metals.

Metals can have corrosive elements that can be reactive

my opinion: in general, that metal came from underground, so there should be no problem. But I suspect a concentration of one metal, such as aluminum, could cause problems with some plants.