By adding a new document this way
curl --location --request PUT 'http://localhost:9200/example1/product/1' \
--header 'Content-Type: application/json' \
--data-raw '{
"title": "example title",
"author": "example author",
"category": "books"
}'
I'm able to add document with "_type": "product", even this is deprecated according to https://www.elastic.co/guide/en/elasticsearch/reference/current/removal-of-types.html.
Is this a feature or bug?