Several people reported problems with their custom plugins no longer being recognized by log4j2. See LOG4J2-673 and this StackOverflow question.
Plugins created before the annotation processor was added to log4j2 (all plugins created with 2.0-rc1 and earlier) may not have a META-INF/org/apache/logging/log4j/core/config/plugins/Log4j2Plugins.dat file.
Previously plugins without this metadata file could still be found if the user specified their custom plugin package(s) in the packages attribute of the <Configuration> element in their log4j2.xml configuration file.
However, since 2.0-rc2, the packages configuration attribute was disabled; users may still specify a value, but log4j2 will no longer use this value to try to load custom plugins. This causes problems for custom plugins built before the annotation processor was added to log4j2, as well as custom plugins that are built in an environment where the annotation processor does not work (for example, most IDEs require some setting changes to enable annotation processing).
This Jira ticket is to reactivate the packages configuration attribute.