Why does named log an error 'disabling RFC 1918 empty zones' when starting up?

In obsolete versions of BIND, named could log this message at startup:

Warning: 'empty-zones-enable/disable-empty-zone' not set:
disabling RFC 1918 empty zones

This behavior was eliminated in BIND 9.9.0. All branches that contained this log message have passed End of Life; if you encounter this message, you should upgrade. Please visit https://www.isc.org/downloads/ for the latest versions of all ISC software.