Found while doing a licence check for T124053#4601302
https://www.wikidata.org/wiki/Special:Version/License/WikibaseLexeme
As with other MediaWiki extensions installed, this should show the software license of this extension.
For example: https://www.wikidata.org/wiki/Special:Version/License/WikibaseQuality
To do this, the extension needs to contain a COPYING file in its root directory, matching the license in the file extension.json.
You are expected to provide a patch in Wikimedia Gerrit. See https://www.mediawiki.org/wiki/Gerrit/Tutorial for how to set up Git and Gerrit.