The _type field can currently be provided in the docs section of simulate pipeline request (as in the example https://www.elastic.co/guide/en/elasticsearch/reference/master/simulate-pipeline-api.html). It would be great to deprecate the use of _type in these requests, making sure to deprecate the relevant methods in the high-level REST response as well.

This API is likely less urgent to address than the others we've encountered, because a typeless request option is already available. In particular, it is already possible to omit the _type field in requests (it looks like a default of _type will be used if a type is not provided).