A reversing ring is used to mount a lens backward on the camera for macro work. When you do that, the rear of the lens becomes exposed, so the lens’s normal front filter or cap arrangement no longer helps protect it.

A reverse-protect ring is not the same thing as the reversing ring itself. Its purpose is protection: it gives you a way to cover or filter the exposed side of a reversed lens. People sometimes make a DIY version by modifying a rear lens cap and adding a step ring, then attaching a filter. Commercial rear-mount adapters that accept filters also exist.

So yes: a reverse-protect style accessory can be used to add protection to a reversed lens, including a filter and sometimes a cap, depending on the design. If not, the alternative is simply to be very careful while shooting.