Chesterton's Fence neatly saves the utterly pointless fences in our lives, regardless of the damage they can cause.
Applied properly, the principle of Chesterton's Fence should provide you with the impetus to observe and learn about the subject. In software, that could involve creating/improving tests, diagramming method/API invocations, monitoring network traffic, etc. As a result of your observations, you should understand the subject deeply enough to determine whether it can be removed safely. If it can't be safely removed, you now have documentation justifying its existence (which may, in some cases, form the basis for a plan to migrate, deprecate, and remove).
If you don't immediately see its use, you should do some more thinking/archeology.
IPv6 designs are only similar to the first half of that statement. If it's in IPv6, it's because someone wanted it there. The people designing IPv6 had almost 2 decades of operational experience running IPv4 to draw from. Things were intentional.