There are a bunch of tagged versions that do not build because of this missing jar.
branch: master
branch: 2.4.1.Final ( which is not provided in the jira version dropdown)
branch: 2.4.0.Final

mvn error msg:
[ERROR] Failed to execute goal on project windup-frames: Could not resolve
dependencies for project org.jboss.windup.graph.frames:windup-frames:jar:2.4.1.Final
: Failure to find org.restlet.jse:org.restlet:jar:2.1.1 in
https://repository.jboss.org/nexus/content/groups/developer/

All of https://repository.jboss.org/nexus/content/groups/developer/org/restlet/jse/
only provided
maven-metadata.xml.md5 Mon Jan 04 06:28:15 EST 2016 33
maven-metadata.xml.sha1 Mon Jan 04 06:28:15 EST 2016 41

https://repository.jboss.org/nexus/content/groups/developer/org/restlet/jse/org.restlet/