"Deprecate" is the appropriate term. For one thing, "depreciate" is seldom, if ever, used to indicate the reduction of an abstract value. (That's off the top of my head; correct me if I'm wrong there.)

More importantly, while it may fit in this particular usage, in common computerese usage, what they're referring to may have nothing to do with a lessening of value, but an admonishment to not use it for some other reason. Most things have been deprecated still function perfectly well, but the designers have decided that you should do it a different way, and that way may have more to do with their notions of style than any technical reason. This is often the case where methods have been moved to new namespaces.

In addition, many of those things that have been deprecated have replacements that have more function, and that is not due to a lessening of function of the old thing, but additional functions for the new thing. For example, PHP deprecated $HTTP_SERVER_VARS in favor of $_SERVER. $HTTP_SERVER_VARS still functions as it always did, but $_SERVER has a greater scope.