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. 
The underlying plugin was never removed and remained to this day, despite
being unmaintained and having known defects.  It is not used anywhere in
Wikimedia Git, nor anywhere else indexed by MediaWiki Codesearch,  and
also not used in any site script or user script (mwgrep). Amazing!
Based on our policy for PHP,  I'm announcing here that the module will
be removed in MediaWiki 1.32.