There are healthy and unhealthy spring mattresses out there, depending on their design. The best spring mattresses on the market are hybrids. Hybrid mattresses feature pocketed coils in their support layers.
This means their coils are all individually wrapped in foam or fabric cases, which makes hybrid support cores much more contouring and motion-isolating. This can offer you the support you need to keep your spine aligned and your body free from pain. Hybrid mattresses are very healthy as a general rule.
The unhealthy spring mattress type is the innerspring mattress. These mattresses feature open coils that are networked together using steel cables. This makes them terrible at isolating motion and contouring to your body. They offer inferior spinal alignment, cause pain, and are generally considered the worst mattress type. So these mattresses are definitely unhealthy.