According to 1 John 2:18, the appearing of many of whom shows it is the last hour?

Answer: Antichrists

"Just as you have heard that the antichrist is coming, so now many antichrists have appeared. This is how we know it is the last hour."

“Children, it is the last hour; and just as you have heard that the antichrist is coming, so now many antichrists have appeared. This is how we know it is the last hour.”— 1 John 2:18 (BSB)