Is my ex-wife still entitled to my annuity if all of our assets have been split?

Most divorce decrees have some "catch all" language that deals with anything not otherwise dealt with in the decree. Some say that any asset not adjudicated belongs to the person in whose name it is held. Some say it belongs equally to both parties. You need to carefully read your Decree (and any other agreements applicable) to see if this language exists. Without such language, you have an asset that was not adjudicated. If the asset was acquired during the marriage, your ex is entitled to half of that portion that was acquired during the marriage.

If the annuity was an omitted asset you must deal with it. Why was it omitted? If it was not disclosed you may have a big problem. If everything else was divided equally I suggest you get a consultation with an attorney to explain the issue of an omitted asset and the Time Rule of Brown.