I was asked what releases do we support in our MediaWiki extension testing. The question is not so much against what releases we can test against, the question is, what gives the best return of investment.
At this moment, the Wikimedia Foundation supports the 1.12 and 1.13 stable releases. People are urged to be as up to date as possible. To stay up to date, people need to know if the extension they have installed will work in these releases.This is the information that we aim to provide.
By concentrating on the stable versions, there is a combination of technical and localisation support for the core MediaWiki product. For extensions there is plenty of room for improvement; the localisation is currently not supported for previous releases and it is less clear what version of the extension works on any given release.
When you want to learn if an extension changed in its behaviour compared to an old version of MediaWiki, you too can test extensions in our environment. When you want to learn if your extension will work against "head", the test environment is there for you as well.