(It’s generally a good idea to start a new thread since many users on the site will ignore threads that started years ago but have recent activity)
Copper prices have been high for a while now, so it’s more likely that your 2015 built building (house, condo?) uses something other than that for the majority of its piping.
I thought that your comment about not using the HWT was a bit odd until I looked at your username. If that gives a clue to your location, I’m curious what kind of water heater you have. If it’s an “on demand” tankless type, then I don’t see any issues with "not using it". On the other hand, if you’ve got a standard HW tank which the water flows through, it really is “in use” even if there is no propane/electricity being used to heat the water. If the tank location is such that you’ve got a large stagnant tank of water at temperatures that promote microbial activity, I’d be a bit concerned. Similarly, an unheated tank filled with water will still be corroding. Do you see the “blue crystals” in any water supply pipe that supplies only cold water? The cold water hose to a washer would likely have a filter, so would be a good place to understand if this was an issue with the hot water supply.