This RFC actually controls the behavior of the advanced find. If it is enabled then you can use History line attributes as filters in advanced find by typing the attribute name (eg: History;Subject). If disabled then typing the attribute does nothing.
Fyi. Searches using history line can severely affects the performance. That was the reason for that RFC, it seems that a user created a query on history line which brought their sd system to a slow grind for everyone else. Fortunately, like George mentioned, it was finally made into an option instead of a complete removal of this feature.