What is this, really?

In the UK, I believe that specific information is the opposite of confidential - water companies are legally required to make maps of public sewers and water mains available for free if you visit their office in person. Many of them also offer maps online, usually for a fee (maybe £50 for a small area; cheap enough that it's worth paying instead of going in person). I think they're often examined when buying a property, so probably a million times a year. And I guess the main reason they're not made available as open data is simply that it would take significant effort and would cut off a source of revenue for the water companies, so there's no benefit for them to do so.

The water regulator has promoted open data and says "We found widespread public support for water companies opening their data, but that water companies have made little progress in opening datasets" (https://www.ofwat.gov.uk/regulated-companies/open-data-in...), so maybe there will be some change but probably not soon, and I see no indication that pipeline maps are a priority (the current progress seems to be primarily about storm overflows, so you can tell which rivers and beaches contain more sewage than normal).

On the other hand there is an in-progress National Underground Asset Register which is "building a digital map of underground pipes and cables", mainly to help people avoid digging into them. It sounds much more comprehensive than just water pipes, and it's based on an open-licensed data model (https://geospatialcommission.blog.gov.uk/2024/08/13/an-in...), but it's explicitly not open data for national security reasons: "We're dealing here with data that can't be seen by everybody from above ground as it's sensitive data relating to gas, electricity and, more importantly, digital data between key parts of our infrastructure. We are security minded in our approach to stop bad actors from accessing this data." (https://www.government-transformation.com/innovation/nuar...)