Hi Francesco,
there is not a date yet for it. We are hard at work with it. You can follow the progress for the first phase on this issue: https://github.com/elastic/elasticsearch/issues/23331 . Most of the search api is done but we are now working in parsing back aggregations. I'd expect the high level client to be released in a few months. Do keep in mind the first version won't support all of the api, but only get, index, bulk, delete, update and search.
Cheers
Luca