In my experience, the most common reason for an ArcGIS license to be obsolete is that the license had an expiry date and that date has passed. If an expiry license has expired, it is obsoleted in Esri's license system, which prevents any further activations using the code.
A non-expiry license can also be marked obsolete for other reasons. For example, say a company has some non-expiry license codes for an older version of a product that is no longer deployed, and they want to make sure no one tries to deploy the older version. The company could obsolete the code to prevent any further activations.
