Board Thread:Support Requests - Getting Technical/@comment-26800159-20151014144601/@comment-168424-20151014173252

Most admins won't know until they try to edit their sitewide JS and find out. So far, it seems like if your wiki did not already use sitewide JS, it's almost certainly locked and that wiki is not eligible for the JS review tool unless you ask for access via Special:Contact/general.

I'm not sure what the criteria is for getting access to the JS review tool, but you can ask.