The documentation of the available mapping parameter for the type completion contains an outdated explanation.
By reading this explanation a user could be mislead into thinking that the standard analyzer still uses the _english_ stopword list by default.

https://github.com/elastic/elasticsearch/blob/6.5/docs/reference/search/suggesters/completion-suggest.asciidoc#mapping

analyzer

The index analyzer to use, defaults to simple. In case you are wondering why we did not opt for the standard analyzer: We try to have easy to understand behavior here, and if you index the field content At the Drive-in, you will not get any suggestions for a, nor for d (the first non stopword).

It looks like the explanation why the simple analyzer is used instead of the standard analyzer is outdated.
In Elasticsearch versions prior to 1.0 the standard analyzer used the _english_ stopword list as default.

https://www.elastic.co/guide/en/elasticsearch/reference/0.90/analysis-standard-analyzer.html

This was changed subsequently but the documentation for the completion suggester is still based on the old version. Now the standard analyzer uses an empty stopword list by default.

https://www.elastic.co/guide/en/elasticsearch/reference/1.3/analysis-standard-analyzer.html

This part of the documentation was written 5 years ago, so it's pretty likely that it didn't get updated to reflect the implementation changes in the standard analyzer.

https://github.com/elastic/elasticsearch/blame/6.5/docs/reference/search/suggesters/completion-suggest.asciidoc#L50