The update site only has 2.9.0 version, the other versions do not appear. Looking at the physical update site on dist/uima/eclipse-update-site/uima-as, in the features and plugins dirs, there are versions for 2.3.1, 2.4.0, 2.4.2, 2.6.0, 2.8.1, and 2.9.0.
The artifacts.jar shows all of these, but the content.jar only has 2.9.0.
The uima-as-eclipse-update-site src/main/resources/category.xml has feature elements for 2.3.1, 2.4.0, and the "current one"; not sure what's needed. Looks like same file in the the core uima eclipse update site project only has current one. needs investigation.