Which sign is not required to be posted at all pools?

The requirement for signage at pools is largely governed by safety standards intended to protect swimmers and inform them about the best practices while using the facility. The sign indicating "Shallow Water - No Diving" is crucial because diving into shallow water can lead to severe injuries. However, while it is technically important, it may not be legally mandated at all pools, depending on local regulations.

In contrast, the "Deep End - No Running" sign serves to remind swimmers of the hazards associated with running near the pool's edges, where slipping could occur. "No Food or Drink" is a common regulation in many swimming facilities to maintain cleanliness and prevent accidents, making it widely required. The sign stating "Pools Closed During Lightning" is also a vital safety precaution, ensuring that swimmers leave the water during stormy weather to prevent drowning or injury from lightning strikes.

Thus, while "Shallow Water - No Diving" is important for safety, it is not universally required at all pools, differentiating it from the other options that are often mandated by local health regulations or safety best practices.