This is true, and very much an issue in maritime archaeology. World War II wrecks in the South China Sea are particularly vulnerable to salvaging for low-background steel (usually by China, although other countries are involved) and there are a lot of ethical and legal issues surrounding this. There are two main challenges to disturbing war wrecks for metal: the first is that, as a rule of thumb, no matter where in the world a warship sank, under international law it is still considered property of its home nation, meaning that ships may be salvaged by companies or countries that have no legal right to salvage them (a great case study for this is the illegal salvaging of the Spanish treasure ship Nuestra Señora de las Mercedes by an American company, Odyssey Marine Exploration.) The second is that many of these sites are war graves, meaning that there is a major ethical dilemma to recovering them, as in order to recover the metal or other contents, what is in essence a graveyard will be disturbed (cf. the recovery of the Gairsoppa by the U.K. government involving, surprise surprise, the American salvage company Odyssey Marine Exploration.) For maritime archaeologists, there is also the third issue that salvage, like archaeology, is destructive, but unlike archaeology, specific material is prioritised over general material culture, meaning that valuable information can be lost in the process.
Tl;dr: pre WWII shipwrecks are often salvaged for metal and it’s a very complex and challenging issue.