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?”” .