Must PVC water pipes be inside the walls?

Haven't seen much PVC anymore for supply -CPVC and PEX dominate. PVC is widely used as drains. In 1 story slab homes much.of the supply is through the floor not the walls. In short, the answer is NO.

PVC, if left exposed to the elements, will brittle and eventually crack. Also, if left unprotected, it can be easily damaged. Therefore, I suggest you hide PVC plumbing inside the wall / ceilings / crawl spaces and install nail shields where ever pipe penetrates lumber. Moreover, where PVC penetrates concrete slab, install foam wrap around the pipe to absorb movement and friction.

Back to you. Milo