The SkinTemplateOutputPageBeforeExec hook is being deprecated to support the upcoming refresh of the vector skin (target 1.35 release - see T60137 for background)
The wikidata.org extension uses the hook to add a link to the site footer, however a new and more direct hook serves that purpose - SkinAddFooterLink
I've provided a patch (untested) to resolve this bug. Thanks in advance for your support with code review!