In general though, if we learned ANYTHING during the supply chain lockups during COVID, it should have been that there is some theoretical minimum set of "stuff" whose production would likely best remain on domestic shores to ensure access and steady supplies in the event of an extended interruption in international shipping.

True....but remember, it's not like Nippon's purchase of U.S. Steel would result in them taking U.S. Steel back with them to Japan. All of their important physical assets would certainly stay here in the states, which means the production would still remain on domestic shores. Just under different ownership. U.S. Steel doesn't seem like the sort of company that would have massive amounts of intangible or non-physical assets that could be "exported" back to Japan (like IP or technology that might violate export controls) - it's a steel company. So other than the scenario that 1pg mentioned, that Nippon might change the product mix to one more favorable to other uses than military equipment, it's hard to see how a change in ownership would affect the amount of domestic production of steel.