Elasticsearch read_only_allow_delete auto setting

Elasticsearch automatically sets "read_only_allow_delete": "true" when hard disk space is low.

Find the files which are filling up your storage and delete/move them. Once you have sufficient storage available run the following command through the Dev Tool in Kibana:

PUT your_index_name/_settings
{
 "index": {
   "blocks": {
     "read_only_allow_delete": "false"
    }
  }
}

OR (through the terminal):

$ curl -XPUT -H "Content-Type: application/json" \
   http://localhost:9200/_all/_settings \
     -d '{"index.blocks.read_only_allow_delete": false}'

as mentioned in your question.