This hook was hard deprecated in 1.35 (T253783) and we should look to remove it in 1.36
https://codesearch.wmflabs.org/search/?q=SkinTemplateToolboxEnd&i=nope&files=&repos=
Certain skins such as Vector dropped support for the hook in T252917 so it's possible functionality will not work in 1.35 (although will not throw errors)
Also impacted:
RelatedLinks, advancedbacklinks, chameleon skin, DarkVector, SemanticWebBrowser, SemanticMediaWiki, erudite, p2wiki, WPtouch, foreground