YARN-5536 aims to replace the XML format for the excludes file with a JSON format. However, it looks like we won't have time for that for Hadoop 3 Beta 1. The concern is that if we release it as-is, we'll now have to support the XML format as-is for all of Hadoop 3.x, which we're either planning on removing, or rewriting using a pluggable framework.
This comment in YARN-5536 proposed two quick solutions to prevent this compat issue. In this JIRA, we're going to remove the XML format. If we later want to add it back in, YARN-5536 can add it back, rewriting it to be in the pluggable framework.