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
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