Now with the OCR text converter option, the only way to quickly search something containing certain word in a caption is to use Tools > Advanced Search which opens additional window and you have to use several drop down menus to find Caption search option.
The UI of digiKam is lacking intuitiveness a lot. Would be better if there is a quick button somewhere near filters to be able to select many of the same (or most widely used/necessary) options as from the settings window.