The former default log file directory

pg_log was the default log file directory in PostgreSQL 9.6 and earlier.

By default, PostgreSQL logs are written to the location specified by log_directory, which defaults to a subdirectory of the data directory. In PostgreSQL 10, the default value for log_directory was changed from pg_log to log, to differentiate it from other directories beginning with pg_ which are essential components of the data directory.

pg_log should not be confused with the former system catalog table pg_log.