18 For I testify unto every man that heareth the words of the prophecy of this book, If any man shall add unto these things, God shall add unto him the plagues that are written in this book:
19 And if any man shall take away from the words of the book of this prophecy, God shall take away his part out of the book of life, and out of the holy city, and from the things which are written in this book.
Sound scary? Well, a clearer understanding will remove any fear that John was condemning Latter-day Saints and their additional revelations.
As usual, context matters. Moses made a similar comment in Deuteronomy. Proverbs mentions a similar concept. And John himself likely wrote his Gospel and epistles after writing Revelation.
No, the canon (New Testament) wasn’t closed with John’s words in Revelation 22.