Our goal is to use the classic similarity for all the indices. I solved my problem using template.
PUT /_template/template_1?pretty
{
"template": "*",
"settings": {
"index": {
"similarity": {
"default": {
"type": "classic"
}
}
}
},
"mappings": {
}
}
https://www.elastic.co/guide/en/elasticsearch/reference/current/indices-templates.html