We now have a stable interface policy for frontend code that more clearly defines how we deprecate MediaWiki code and what wiki-based code (e.g. gadgets and user scripts).
If you work on extensions or MediaWiki core, or wiki-based code e.g. gadgets/user scripts, please read the policy page at
https://www.mediawiki.org/wiki/Stable_interface_policy/frontend so that you are aware of your responsibilities and please circulate it with your teams/wiki-communities.
This document is intended to be a living document that evolves with our shared needs over time, so this should not be seen as a perfect finalized document and you are welcome to pose questions and adjustments to the policy at https://www.mediawiki.org/wiki/Talk:Stable_interface_policy/frontend
Thank to you the many contributors who have helped build this policy through their feedback and suggestions.