If we are going to support API versions on the order of ~10 years, we will have a lot of parts of the server code that exist only to support old API versions. It would be very helpful to be able to somehow clearly track this old code.

As a possible future scenario, an engineer is looking at some code, thinking that it's the current way things work, it would be good to make it clear that the code is no longer relevant.

Another scenario is being able to clean up the codebase easily (or more easily) by identifying / flagging what can be removed after an old API version is no longer supported. It is unlikely that we will otherwise remember years later what code can be safely removed from the codebase, and code often moves around and may be adjusted over the years.