Binlog versions smaller than 4 had been deprecated before MySQL-5.0.
The compatibility code was removed in this patch.

Start_event_v3 and Start_log_event_v3 are removed from the code.
Some of the code in Start_event_v3 and Start_log_event_v3 were moved
into Format_description_event and Format_description_log_event.

While START_EVENT_V3 type is not removed, it is just a placeholder now.
Server and mysqlbinlog will not recognize START_EVENT_V3 after this patch.