Deprecate Bool query's adjust_pure_negative parameter?

[markharwood]

Is there ever any good reason to have adjust_pure_negative set to false?

We don't think so which is why we don't document this flag and yet it confuses people when it appears.

If there is a use case where users need this option we could keep it as an escape hatch but should instead work to remove examples of requests that needlessly include the default value of adjust_pure_negative:true.
I could only find one example of this in Kibana code (an ML example) but in elasticsearch we do serialize Bool queries to XContent with the default value.

[elasticmachine]

Pinging @elastic/es-search (:Search/Search)

[markharwood]

We discussed this internally and decided that we shouldn't deprecate this flag but should

1. Remove the rendering of the default value in toXContent calls (see BoolQueryBuilder - remove adjust_pure_negative:true from XContent rendering #49543) and
2. Add a small note to bool query docs describing the flag (although probably very de-emphasised because the vast majority of users will only want to use the default choice)