https://www.youtube.com/watch?v=kAzKts6Wp1Q&t=55s
If your water heater is malfunctioning and not heating up all the way to 140 degrees but it's still heating up to e.g. 100 degrees then it's a breeding ground for listeria. The temperature difference changes the solubility of minerals and salts dissolved in it as well as changing how quickly everything even slightly volatile leaches into the water.
Does anyone think the inside of a ~20 year-old cold water feed pipe is going to look shiny and "clean"?
Go take an electron micrograph of the bacteria on your skin, or use a microscope to look closely at the living things inside your bed, in your sofa, or on your pet.
Then take a deep breath, check the causes of death tables if you really have to, and remember that - given we're all intelligent humans - none of these "eww, gross!" things really matter in the overall scheme of things.
I'm not suggesting that those deposits are going to kill you or anything, but if things go wrong and the water isn't heated enough those deposits make a great substrate for harmful bacteria to grow.
Ideally you shouldn't use hot water for consumption even though normally it's safe. It also tastes different than cold water because of the change.