Hello all!
After a brief period for final comments (thanks everyone for your input!), the Stable Interface Policy is now official. You can read it here:
https://www.wikidata.org/wiki/Wikidata:Stable_Interface_Policy
This policy is intended to give authors of software that accesses Wikidata a guide to what interfaces and formats they can rely on, and which things can change without warning.
The policy is a statement of intent given by us, the Wikidata development team, regarding the software running on the site. It does not apply to any content maintained by the Wikidata community.