This is an announcement for a breaking change to the pageterms submodule of the query API module, which only affects Wikibase repository wikis. If you do not use that API module, or only use it on client wikis (e. g. Wikipedias) and not on repository wikis (Wikidata, Wikimedia Commons), you can ignore this message.
For years, the pageterms API module has served a double role: on client wikis, it returned the “terms” (Label, Description, Aliases) of the Wikidata Item linked to the given page(s), whereas on repo wikis, it would return the terms of the Item (or other Entity) on that page itself. For example, querying for the Label of Wikipedia:Village pump on English Wikipedia would return “Project:Village pump” (the Label of Q16503), but querying for the Label of Wikidata:Project chat on Wikidata would not return anything, even though that page is linked to the same Item – you would have to query for the Label of Q16503 instead. This behavior is inconsistent and also mixes repo and client concerns in a way that makes the Wikibase code harder to maintain.
To resolve this, we introduced a new entityterms API module (a submodule of the query module, just like the pageterms module) which has the same behavior as the pageterms module currently has for Item (or other Entity) pages, and which is only available on repo wikis. If you want to get the terms of Q16503, you can now use action=query&prop=entityterms&titles=Q16503 instead of action=query&prop=pageterms&titles=Q16503. (You can also use wbgetentities, which gives you much more control over the returned data; pageterms/entityterms may be faster and can also be combined with other submodules of the query module.) On or shortly after 5 August 2020, we will remove the special repo behavior of the pageterms module, and it will then behave just like it always has on client wikis, and return the terms of the Item linked to a page, not the terms of the Item (or other Entity) on a page. (Because the new API module is already available on Wikidata, and you can start using it immediately, we are not making this pageterms behavior change available on Test Wikidata significantly before that date.)
If you have any issue or question, feel free to leave a comment at T257658. For more information, see also T115117, T255882 and T256255.