There's nothing to suggest that the scope of whatever John was writing included the past especially in the verse you have quoted.
"Things you have seen..." are things John was seeing in the visions. The very first verse has explained it:
Rev 1:
1The revelation from Jesus Christ, which God gave him to show
his servants what must soon take place. He made it known by sending his angel to his servant John,
2who testifies to everything he saw—that is, the word of God and the testimony of Jesus Christ.
The issue about Herod killing the young boys was not a vision to John, he must have witnessed it first hand because he must have been young when it happened and it was all over the news- so how can it be a vision? Even if he wasn't around, he must have heard it as 'breaking news', something that was even prophesied "Rachel weeping for her young ones.."
Jer 31:15
This is what the LORD says: “A voice is heard in Ramah, mourning and great weeping,Rachel weeping for her children, and refusing consolation, because they are no more.”
Matt 2:
14So he got up, took the child and his mother during the night and left for Egypt,
15where he stayed until the death of Herod. And so was fulfilled what the Lord had said through the prophet: “Out of Egypt I called my son.”
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16When Herod realized that he had been outwitted by the Magi, he was furious, and he gave orders to kill all the boys in Bethlehem and its vicinity who were two years old and under, in accordance with the time he had learned from the Magi.
17Then what was said through the prophet Jeremiah was fulfilled:
18“A voice is heard in Ramah,
weeping and great mourning,
Rachel weeping for her children
and refusing to be comforted,
because they are no more.”
John, who witnessed this prophesy fulfilled could not come again in his old age and claim to prophesy or reveal what has already been revealed.