What happened to pg_xlog? There uesd to be a directory called pg_xlog that stored all of the WAL logs in PostgreSQL? Part of restoring a basebackup under the archiving scheme required me to copy the WAL into DATA_DIR/pg_xlog. What happened to this directory?
PostgreSQL 10+

In PostgreSQL 10 the WAL directory was renamed from pg_xlog to the easier to remember pg_wal.


Pg 10: WAL Internals (you can click the top for 9.6 to see the older version before the docs were updated to reflect the change).
New in postgres 10: Renaming of "xlog" to "wal" Globally (and location/lsn) describes the change in more details.