What can you say about Matthew 17:21, which has been removed from recent versions and translations?

Your inquiry about the aforementioned "missing" passage is very crucial. There are several passages in the Bible that are said to have been removed by modern translations but retained by King James translators. Are the modern scholars dishonest in doing this? Are they perverting the Bible because of their actions? The quick answer is no, they are not. 

We must recognize that in biblical Greek translation, the earlier the texts, the more accurate they are. As a result, the quantity of manuscripts will not determine accuracy, but rather the age of the document based on dating. Because a number of recently discovered manuscripts predate those on which the King James translation based their rendition, we must rely on recent findings that were unavailable in the 17th century.   

If Matthew 17:21; 18:11; 23:14; Mark 9:44, 46; 11:26; 15:28; Luke 17:36; 23:17; John 5:4; Acts 8:37; 15:34; 24:7; 28:29; and Romans 16:24 are not found in the earliest manuscript, it is likely that the paragraph was added by some scribes to insert their comments into the Scriptures. 

Despite this potential, many recent versions continue to insert them in the footnotes.