top.h has an obsolete comment about the use of _unfiltered.
/* Serious error notifications */
struct ui_file *m_gdb_stderr;
/* Log/debug/trace messages that should bypass normal stdout/stderr
- filtering. For moment, always call this stream using
- *_unfiltered. In the very near future that restriction shall be
- removed - either call shall be unfiltered. (cagney 1999-06-13). */
+ filtering. */
struct ui_file *m_gdb_stdlog;
/* The current ui_out. */