It had a good run. It's the version that convinced many campuses and organizations that federated authentication was doable, and would provide significant value to their community.
But, all good things must come to an end. June 30, 2010, marks the last day that the Shib team will offer support for v1.3. At that time, it will transition to the "unsupported" category; see http://shibboleth.internet2.edu/shib-which-version.html.
Unsupported is defined as "We are willing to entertain questions, but in many cases no resolution will be possible without upgrading." Going forward, NO security patches will be provided (even for reported security problems that are severe). As knowledge about v1.3 fades from memory, fewer and fewer questions will be answered.
At this point, the Shib team VERY STRONGLY encourages any site still using v1.3 to upgrade to the current release as soon as possible, where "as soon as possible" means "put this upgrade at the very top of your project list." While v1.3 was significant from a historical perspective, it has numerous issues and sites should immediately transition to the current version.