HDFS-4171 made the invalid assumption that there is a common limit on user names at the UNIX level. Almost all modern systems, when running under a 64-bit kernel, use a pointer instead of a char array in the passwd struct. This makes usernames essentially unlimited.
Additionally, IIRC, the only places where HDFS and the OS interact where the username is touched is during group lookup.
This limit is artificial and should be removed. There is a very high risk that we will break users, especially service accounts used for automated processes.