Why submarine propellers became less visibly secret

Why do modern navies seem less concerned about hiding submarine propellers?

Threat perceptions changed and many submarines now use pump-jets, but propulsor geometry still matters because it can help identify a boat's acoustic signature.

Propeller geometry once received intense protection because analysts could connect cavitation noise with an individual submarine. A period of lower perceived threat may have relaxed some visible precautions, while modern boats increasingly use pump-jets rather than exposed conventional screws. Concealment could tighten again if acoustic intelligence becomes a sharper concern.