Deleted Verses in the New World Translation

There is a Catholic website at http://www.catholicapologetics.info/scripture/translations/neworld.htm that details many of the Bible verses deleted from the New World Translation Bible. I wouldn't even bother with this drivel if it wasn't for the fact that I see this page shared a lot.

One of the first passages this website notes as being deleted is Matthew 17:21.

Take note of the image above. Those captions are from three different Catholic Bibles and all of them have "deleted" the same passage that the NWT Bible did.

Why?

The oldest manuscripts did not have these verses. The King James Version Bible was released in 1611 and it was based primarily on late manuscripts, not older ones. Older mss were discovered AFTER the release of the KJV Bible. Codex Sinaiticus, one of the four great ancient uncial codices wasn't discovered until the 1840's. The value of the Codex Vaticanus and Alexandrinus was not realized until quite some time after 1611.

There is even a wikipedia entry for the deleted verses at https://en.wikipedia.org/wiki/List_of_New_Testament_verses_not_included_in_modern_English_translations

So it is not that certain verses were deleted, it is just that they were added at a later date.