[Update: This post got some well-deserved pushback. Thanks to Patrick Durusau, Lars Marius Garshol and Jack Park for the feedback – and sorry for the controversial headline. I’ve added “(as a standard)” and did some editing to make clear that people still use Topic Maps.]
I’m a fan of Topic Maps – the very well-thought-out Topic Maps Data Model standard with an XML serialization called XTM (XML Topic Maps) dating back to the year 2000. (See also the Topic Maps Reference Model, TMRM).
Even as a fan, I must admit that the Topic Maps standards are dead. They have never been widely adopted, and the key contributors have long moved on. Measured by the value you expect to get out of a successful standard – good visibility, adoption, interoperability, tooling, ongoing development – Topic Maps haven’t been the success we were hoping for.