"Content type detection for rest requests is deprecated. Specify the content type using the [Content-Type] header." in elasticsearch_deprecation.log.
It seems to be a error stated here: https://github.com/olivere/elastic/issues/511
Where Content-Type header should be set to application/x-ndjson.
Extension:CirrusSearch might need to change it's code accordingly, since the api of Elasticsearch changed.
I am using:
PHP 7.0.27-1~dotdeb+8.1 (fpm-fcgi)