yes it's true. If you use a multivolt PC on any other voltage above 120, it won't work on 120 anymore since there's a fuse type strip that burns away and opens the circut to the 120V tap. The 208+ tap is still okay on 120V since the 120V flowing though it doesn't do anything but at 208 or higher, there's no going back.
to prevent frying. the 208-277V circut can handle 120V, but the 120V circut can't handle a higher voltage. I think the newer electronic ones can be used on 120V after though. i think there's a fuse type thing that burns away and opens the 120V circut when a higher voltage is applied, so maybe it's possible to revive a PC to work on 120V. the 208-277 circt is always closed though
Well, you could make an electronic circuit that auto-senses the voltage but that would drive up costs for a feature that won't be used often. PCs are seldom reused so it makes sense to the company to make the voltage selection one-shot.