Module Uninstall Validators and Lazy Services
It's a property of services classes provided by Symphony's dependency injection framework. Per its docs on lazy services:

In some cases, you may want to inject a service that is a bit heavy to instantiate, but is not always used inside your object. For example, imagine you have a NewsletterManager and you inject a mailer service into it. Only a few methods on your NewsletterManager actually use the mailer, but even when you don’t need it, a mailer service is always instantiated in order to construct your NewsletterManager.

In other words, proxy classes are quick-loading dummy stand-ins until the actual class methods are needed to do the heavy lifting. In this specific case, the Book uninstall validator isn't useful until it's needed during the book module uninstall so there's no need to fully instantiate the validator service.