Extensions are (supposed to be) documented on a page on MediaWiki. This documentation includes Template:Extension to provide a summary of important information about the extension, including the "required version of MediaWiki".
This version field is inadequate: does specifying "1.19+" mean that the extension's master branch is intended to be compatible with MediaWiki 1.19, or that the extension has release branches going back to REL1_19 while the extension's master branch is only intended to be compatible with MediaWiki core's master branch, or perhaps some middle ground?
Some extensions have solved this by including detailed text in the existing "version" parameter, such as "1.26+ . Flow master is only supported with core's master; otherwise, use matching branches (e.g. Flow REL1_26 with core REL1_26, or matching WMF release branches).", but this is currently done inconsistently and infrequently.
Who would benefit
Developers making updates to extensions for a core change in compliance with the new deprecation policy will no longer have to guess at whether backwards compatibility with older versions of MediaWiki core should be maintained.
Developers of extensions that don't maintain compatibility with old versions of MediaWiki won't have unnecessary and sometimes complex backwards-compatibility code added.
Developers of extensions that do maintain compatibility with old versions of MediaWiki won't have to -1 or -2 as many changes that break backwards compatibility.
- Update Template:Extension with parameters to indicate the MediaWiki versions for which release branches exist versus the MediaWiki versions the extension's master branch maintains compatibility with.
- Update existing extension pages to use these parameters, which may involve determining the master-compatibility policy where it's not already specified.