While Rev 22:18-19 has been refuted by Latter-day Saint apologists ad nauseam, and while even many vehement critics of the Church reject it as a valid proof-text against modern public revelation and/or Sola Scriptura, it still is raised by some (often the more ignorant) critics of the Church. Here is one recent example:
If we will only stick to God’s
Word in the Bible, it will be a shield to protect us from the lies that not
only lead us astray, causing hurt and harm, but also keep us from being ripped
off by false teachers. Scripture is our shield. As we just read, God says not
to add to His words, or He will rebuke us, and we will be proven liars. Bingo!
With all due respect, that is exactly what has happened to Joseph Smith. Here
is more from Revelation 22:18-19:
“I testify to everyone who hears
the words of this book: If anyone adds to them, God will add to him the plagues
which were written in this book. And if anyone takes away from the words of
this book. God will take away his part in the tree of life and in the holy
city, which are written in this book.”
God’s message is very clear: Don’t
add or take away from Scripture; leave it alone because I have put my stamp of
approval on it 2,500 times. Isn’t that good enough/ Why do we have to keep
going somewhere else? Just like those Old Testament stories of the people
constantly straying and coming back, only to stray again. Even today we are
looking for other ideas, theories, and prophets (outside the Bible) to guide
us. But why can’t we just get into the Bible and enjoy it straight from the
mind of God? (Bill Crone, Mormonism and the Occult Behavior of Joseph Smith [World
Religions Cult and the Occult; Pahrump, Nev.: Get a Life Ministries, Inc., 2020],
224)
To see a thorough refutation of this text as well as the (comparatively speaking) "stronger" texts such as 2 Tim 3:16-17, see:
Not By Scripture Alone: A Latter-day Saint Refutation of Sola Scriptura