Currently, only read support is provided to indexes created in 5.x. As someone who used one of their pre-GA releases, they previously had a flag to enable multiple mapping types.
> Elasticsearch 6.x
>> Indices created in 5.x will continue to function in 6.x as they did in 5.x.
[1] https://www.elastic.co/guide/en/elasticsearch/reference/6.0/...