You can access the latest development snapshot by using the Maven repository https://repository.apache.org/snapshots and the current SNAPSHOT version. Generally, the master branch will use the next patch version as its snapshot version regardless of what the next actual version of Log4j will be. For example, if the latest release were 2.0, then master would be using the version 2.0.1-SNAPSHOT. Always verify with pom.xml in the master branch as described in the source repository page.

Maven users can add the following to their pom.xml to enable snapshots:

<repositories>
  <repository>
    <id>apache.snapshots</id>
    <name>Apache Snapshot Repository</name>
    <url>https://repository.apache.org/snapshots</url>
    <releases>
      <enabled>false</enabled>
    </releases>
  </repository>
</repositories>

via https://logging.apache.org/log4j/2.x/maven-artifacts.html