What i wanna do:
I have a dashboard that uses two diferent index patterns, and i have created a filter that only restrain result from one index pattern. After enable that filter some visualizations send a message No result found.
What i think that ignoreFilterIfFieldNotInIndex set at true do :
I think that will ignore the filter for indices that don't have a field that is filtered, or, at least, ignore the filter.
Related topic that i found :
Kibana without x-pack : 6.6.2