Regarding the question about use_field_mapping warnings in the linked post above:
"[use_field_mapping] is a special format that was only used to ease the transition to 7.x. It has become the default and shouldn't be set explicitly anymore."
I also get these warnings on the nodes in our cluster. I understand that use_field_mapping should no longer be used, but how can I find out where it is used? What should I look for? I have no recollection of ever using this in any scripts that index data to Elasticsearch.