Hi,
The 'jquery.expandableField' created in 2011 for use by the Vector skin (back when skins were still part of core). More specifically, it was part of an experimental option brought in via the Vector *extension*. It was never enabled by default and the option was removed in 2013. [1]
The underlying plugin was never removed and remained to this day, despite being unmaintained and having known defects. [2] It is not used anywhere in Wikimedia Git, nor anywhere else indexed by MediaWiki Codesearch, [3] and also not used in any site script or user script (mwgrep). Amazing!
Based on our policy for PHP, [4] I'm announcing here that the module will be removed in MediaWiki 1.32.
Patch: https://gerrit.wikimedia.org/r/#/c/mediawiki/core/+/453454/
-- Timo
[1] https://gerrit.wikimedia.org/r/#/c/mediawiki/extensions/Vector/+/84195/ [2] https://phabricator.wikimedia.org/T29647 , https://phabricator.wikimedia.org/T48515 [3] https://codesearch.wmflabs.org/search/?q=jquery.expandableField [4] https://www.mediawiki.org/wiki/Deprecation_policy