User blog comment:Rappy 4187/Technical Update: February 5, 2013/@comment-452-20130205180732/@comment-452-20130206140120

PedroM, you apparently joined in April 2012 - after the new editor was introduced, so you're missing some information.

The previous editor had the category module enabled for source mode (with the option to disable it, obviously). They removed this functionality when the new editor was introduced, and it has taken them a year to restore it. (Although I re-enabled it with javascript a while ago anyway)

Today's change has simply restored how it used to be.

(Note: I'm uncertain about the exact length of time, trying to look it up now - edit: it has been well over a year since the new editor was introduced)