You say ONLY_FULL_GROUP_BY was late added. But there are posts talking about this setting in 2002.
http://dev.mysql.com/doc/refman/5.0/en/server-sql-mode.html#...
> Do not permit queries for which the select list or (as of MySQL 5.0.23) HAVING list refers to nonaggregated columns that are not named in the GROUP BY clause.
ONLY_FULL_GROUP_BY was added in 4.0 but only applied to select list columns, the 5.0.23 mentioned above applies to the HAVING clause list not allowing them as well:
http://dev.mysql.com/doc/refman/4.1/en/server-sql-mode.html#...