Dan from Google told Alex from GOS blog:
Yes, it’s been deprecated. Why? Because too few people were using it to make it worth the time, money, and energy to maintain. In truth, although I sometimes disagree with the operator changes, I happen to agree with this one. Maintaining ALL of the synonyms takes real time and costs us real money. Supporting this operator also increases the complexity of the code base. By dropping support for it we can free up a bunch of resources that can be used for other, more globally powerful changes.
Some folks are upset by the featuring going away - it was a great way to do keyword research for many marketers.
Forum discussion at Google+.
Update: Matt Cutts shared another reason why this was removed.