Once this is done, Wikidata should be updated to use this client.

Change 404064 had a related patch set uploaded (by Smalyshev; owner: Smalyshev):
[mediawiki/core@master] Add SPARQL client to core

https://gerrit.wikimedia.org/r/404064

Change 404064 merged by jenkins-bot:
[mediawiki/core@master] Add SPARQL client to core

https://gerrit.wikimedia.org/r/404064

I believe that adding a pointer to the respective documentation accompanying this new feature will be great for future reference.

Do we have a place for documenting internal APIs? I haven't found where it could be added besides the regular phpdocs.

Well, this client should probably be in a library soon / already anyway?
Said library should contain the docs, It might even be worth marking it for internal use until we put it in a library.

I must have misunderstood the newsletter sent out to the public. I thought that is is about a user facing something. Anyways, documenting stuff is always good and I know from experience that this is painful.

In T185127#4007262, @Smalyshev wrote:

Do we have a place for documenting internal APIs? I haven't found where it could be added besides the regular phpdocs.

Usually Manual:<classname> on mediawiki.org. E.g. https://www.mediawiki.org/wiki/Manual:User.php

Some people also use the docs/ directory in core, but that's less common.

Well, this client should probably be in a library soon / already anyway?

Depending on how soon you can make HTTP client a library, which depends on Status, which in my book spells out "not soon". See more discussion on the matter in https://gerrit.wikimedia.org/r/c/404064/

@Legoktm: thank you, I wasn't aware of that system. I will add docs there for SPARQL and will keep it in mind for other stuff too.

Added some docs to https://www.mediawiki.org/wiki/Manual:SparqlClient.php.