Matthew 16:28

I tell you the truth, some who are standing here will not taste death before they see the Son of Man coming in his kingdom.

Mark 9:1

And he said to them, "I tell you the truth, some who are standing here will not taste death before they see the kingdom of God come with power.

Luke 9:27

I tell you the truth, some who are standing here will not taste death before they see the kingdom of God

Re saying John wouldn’t die, that’s actually an inference but a not uncommon theological belief among evangelicals. John 21:20-23

“20 Peter turned and saw that the disciple whom Jesus loved was following them. (This was the one who had leaned back against Jesus at the supper and had said, “Lord, who is going to betray you?”) 21 When Peter saw him, he asked, “Lord, what about him?”

22 Jesus answered, “If I want him to remain alive until I return, what is that to you? You must follow me.” 23 Because of this, the rumor spread among the believers that this disciple would not die. But Jesus did not say that he would not die; he only said, “If I want him to remain alive until I return, what is that to you?”” .

I believe OP is referring to Matthew 24:34 (Jesus is speaking answering the question of when he will return): "Truly I tell you, this generation will not pass away until all these things have taken place."

It never explicitly mentions a “rapture”, but describes an event that evangelicals have interpreted to mean one will happen. Not all denominations believe in the idea…per usual amongst the many diverse belief systems of Christianity.

The most popularly referenced text is this:

In 1 Thessalonians 4:16-17 (New International Version), it is written:

"For the Lord himself will come down from heaven, with a loud command, with the voice of the archangel and with the trumpet call of God, and the dead in Christ will rise first. After that, we who are still alive and are left will be caught up together with them in the clouds to meet the Lord in the air. And so we will be with the Lord forever."