The javascript: in URLs has from my reading on the internet been around for a very long time while not being an actual standard. The protocol is currently part of the HTML spec. When was this introduced into the HTML spec? I did a cursory search of the HTML4 standard and did not find it. I also found this IETF draft from 2010 proposing the resource identifier scheme.
It looks like Ian Hickson added the "Javascript Protocol" section to the spec (basically just to indicate an intent to define it) on Thu, 25 Jan 2007 20:12:17 +0000
(Earlier versions do mention "javascript: URIs", but mostly in comments, questions, and things to be done.)
The commit of Thu Mar 1 22:41:49 2007 +0000 ("behold: the javascript: URI") is the first to provide detailed semantics, although it relies on an external "JSURI" reference.
The commit of Fri Nov 15 15:56:16 2013 +0000 ("Move javascript: processing entirely into HTML") is the first that doesn't need the "JSURI" reference.