Use the deprecated annotation to identify methods, classes, exceptions, enums, interfaces, or variables that can no longer be referenced in subsequent releases of the managed package in which they reside. This is useful when you are refactoring code in managed packages as the requirements evolve
Deprecated annotation is used in the context of packages in salesforce and it suggests the class, exceptions, enums, interfaces, variables, or methods that can no more in use or will not be distributed in further releases if its part of a package.