This was due to RESTEasy upgrade to v4, which for instance dropped the artefact resteasy-jaxrs, which the quickstart depends.

Please note that the quickstart is currently disabled from the project build (root pom.xml) and dist (dist/src/.../assembly.xml)

Also note that similar happened to resteasy-jaxrs-client quickstart, which was already fixed by https://github.com/wildfly/quickstart/pull/522