According to Osho, the claim is misguided: calling reality or a 'soul' 'unstained' already presupposes the possibility of stain. The ultimate is beyond such opposites, and even beyond the label 'soul.' Naming freezes a fluid suchness into duality; dropping labels loosens the mind's knot and reveals what-is without conceptual distortion.
If you call it a perfectly clean 'soul,' you create the idea of dirt; drop the label and just see what is, and the mind settles.