User blog comment:Kirkburn/Technical Update: September 2, 2015/@comment-3388044-20150904013930/@comment-13301-20150904173509

@SpikeToronto It's a technology limitation imposed by Wikia.

If you know a little of programming, you'll probably understand from this code that every user subpage ending in .js that's not a skin name, or global.js in community central, will be disallowed

Basically, Wikia is imposing that limitation, where MediaWiki itself doesn't impose any (except the ability to edit other user's personal JS or CSS pages)