Prophesy is proclaiming God's words. So people that prophesied in the past usually just said, "thus saith the Lord." In those days, the scriptures weren't complete, so a lot of prophesy of the Lord was of future events. Today, exhorting each other in the written scriptures is like prophesying. It's different than saying, "thus saith the Lord." Nowadays you cite book/chapter/verse. It's the same thing as proclaiming God's word because the scriptures are his words.
But we believe that God's word has been completed from Genesis to Revelation. That doesn't mean people aren't given revelations. Revelations are the spiritual understanding of certain scriptures. I say certain because to have complete revelation of all the dark sayings of the entire scriptures is daunting.
So, if you're Joseph Smith and you believe you found angelic tablets written in angelic language that you spend 3 or however many years discerning and then telling others, I would think those types of prophesies outside of God's completed word are of a "perverse spirit," not the Lord.