We still provide example configurations for customizing the search index in dspace.cfg:
These lines are only for the deprecated Lucene search index. Discovery has its own way of configuring search fields (in a nutshell: write a new search filter for each field, and then tell Discovery to use those filters):
The presence of the old examples in the dspace.cfg file, and the current Discovery documentation, create a situation where people who upgrade from a Lucene search DSpace (3 or less) to a Discovery search DSpace (4 or greater) will have no obvious way to migrate their search customizations. Our example configurations and documentation should be clearer on this topic.