So the saying among the brethren that John would not die. John describes himself as giving testimony in final words of Book of John Chapter 21.

20 Then Peter, turning about, seeth the disciple whom Jesus loved following; which also leaned on his breast at supper, and said, Lord, which is he that betrayeth thee?

21 Peter seeing him saith to Jesus, Lord, and what shall this man do?

22 Jesus saith unto him, If I will that he tarry till I come, what is that to thee? follow thou me.

23 Then went this saying abroad among the brethren, that that disciple should not die: yet Jesus said not unto him, He shall not die; but, If I will that he tarry till I come, what is that to thee?

24 This is the disciple which testifieth of these things, and wrote these things: and we know that his testimony is true.

25 And there are also many other things which Jesus did, the which, if they should be written every one, I suppose that even the world itself could not contain the books that should be written. Amen

PETER is not a good witness because he denied Jesus 3 times before Jesus death. John never denied Jesus and was at the cross.

John is considered to be exiled to Patmos, undergoing a time of persecution under the Roman rule of Domitian. Revelation 1:9 states: “I, John, both your brother and companion in tribulation … was on the island that is called Patmos for the word of God and for the testimony of Jesus Christ.” Adela Yarbro Collins, a biblical scholar at Yale Divinity School, writes:

Early tradition says that John was banished to Patmos by the Roman authorities. This tradition is credible because banishment was a common punishment used during the Imperial period for a number of offenses. Among such offenses were the practices of magic and astrology. Prophecy was viewed by the Romans as belonging to the same category, whether Pagan, Jewish, or Christian. Prophecy with political implications, like that expressed by John in the book of Revelation, would have been perceived as a threat to Roman political
power and order. Three of the islands in the Sporades were places where political offenders were banished.
(Pliny, Natural History 4.69–70; Tacitus, Annals 4.30) [13]

John was allegedly banished by the Roman authorities to the Greek island of Patmos, where, according to tradition, he wrote the Book of Revelation. According to Tertullian (in The Prescription of Heretics) John was banished (presumably to Patmos) after being plunged into boiling oil in Rome and suffering nothing from it. It is said that all in the audience of Colosseum were converted to Christianity upon witnessing this miracle. This event would have occurred in the late 1st century, during the reign of the Emperor Domitian.

I have never met John in person not yet but that does not mean that he does not exist. If I have the testimony of John …. I have the words that Christ gave to witness John. Christ prays for those who believe in him through the testimony of John.

20 Neither pray I for these alone, but for them also which shall believe on me through their [ witnesses ] word;