The _ignored field indexes and stores the names of every field in a document that has been ignored because it was malformed and ignore_malformed was turned on.

This field is searchable with term, terms and exists queries, and is returned as part of the search hits.

For instance the below query matches all documents that have one or more fields that got ignored:

GET _search
{
  "query": {
    "exists": {
      "field": "_ignored"
    }
  }
}

Similarly, the below query finds all documents whose @timestamp field was ignored at index time:

GET _search
{
  "query": {
    "term": {
      "_ignored": "@timestamp"
    }
  }
}