For any book of the Bible, but especially the New Testament, there are many conservative scholars whose audience is primarily devotional or pastoral and would prefer to understand the scriptures as discrete, synchronic works—each written by the person whose name appears in the title if at all possible, although that ship has long sailed for the most part.

As John Ashton notes, the lack of manuscript evidence is often cited by scholars in order to dismiss proposals for redaction and interpolation of texts. Chapter 21 of John is so clearly a secondary addition to the book, however, that this objection cannot be sustained. https://www.wolfson.ox.ac.uk/content/1140-dr-john-ashton
Once this rampart has been breached the enemy is within the gate: there can be no further defence of integral unity as a matter of principle, and the familiar argument that there is no manuscript evidence, either here or elsewhere, of editorial insertions or modifications loses all its force. (p. 42)

Some of the best reasons for considering John 21 (the “Johannine Appendix”) to be a later addition are the following:

• The story appears to end with a “resounding conclusion” in 20:30–31: Now Jesus did many other signs in the presence of his disciples, which are not written in this book. But these are written so that you may come to believe that Jesus is the Christ, the Son of God, and that through believing you may have life in his name.
• Chapter 21 makes the unnamed “Beloved Disciple” out to be the author of the book (21:24). This is not stated or suggested at any previous point. At the same time, the narrator takes pains to dispel rumours that the Beloved Disciple (whoever he was) would not die. As Bultmann puts it, “the fiction that the author himself puts himself forward here as identical with the Beloved Disciple, and at the same time wishes to attest his own death [v. 23] is quite unbelievable” (quoted in Ashton, 43).
• The style of the Johannine Appendix and nature of the miraculous catch of fish are considered to be quite different from the rest of John.
• The chapter is hard to understand from a narrative point of view. In chapter 20, Jesus appears to the disciples in a locked room, commissions them, and bestows the Holy Spirit upon them. And then in chapter 21, they go back to their old jobs as Galilean fishermen—which, unlike the Synoptics, is not even hinted at in John when the disciples are first called.

Scholars almost universally consider John 21 an addition to an earlier edition of the Gospel. Several scholars have argued that ch. 21 belongs to the same compositional stratum as the rest of the Gospel, but C. K. Barrett admirably summarizes the philological evidence that the Evangelist was not the author of ch. 21 (The Gospel According to Stjohn: An Introduction with Commentary and Notes on the Greek Text [London: SPCK, 1958], 479-80). See also HartwigThyen, “johannes 13 und die ‘Kirchliche Redaktion’ des vierten Evangeliums,” in his Studien zum Corpus lohanneum (WUNT 214; Ti.ibingen: Mohr Siebeck, 2007), 30-31, esp. n4. For a judicious treatment of the topic and a defense of two compositional strata see especially Francis J. Moloney, “john 21 and the johannine Story,” in Anatomies of Narrative Criticism: The Past, Present, and Futures of the Fourth Gospel as Literature (ed. Tom Thatcher and Stephen D. Moore; SBLRBS 55; Atlanta: Society of Biblical Literature, 2008), 237-51.

The last serious addition to the text of John (7:53 – 8:11) is first attested in manuscripts from the fi fth century, though the addition could be earlier. It cannot have been part of the gospel as originally published. It interrupts the narrative of John 7 and 8 and is not found in the early manuscript tradition.
**An earlier addition, attested uniformly by our manuscript tradition, is John 21. It is identified as an addition by a critical examination of the text: **

• (1) John 20:30 – 1 reads like the original conclusion to the gospel.
• (2) Chapter 21 is distinguishable from John 1 – 20 on grounds of style, vocabulary, and content.
• (3) The “ we ” of John 21:24 implies that chapter 21 was added by hands other than that of the disciple who bore witness in John 1 – 20.
• (4) The evidence suggests that the perspective of John 21 is an extension of the Johannine tradition, but in a way unprepared for by John 1 – 20. There Peter and the Beloved Disciple were paired in several scenes, always to the advantage of the latter. In John 21 the presence of the Beloved Disciple is noted, but Peter plays the leading role, and the fi nal scene portrays Peter with Jesus in a way that restores Peter after his threefold denial of Jesus. Jesus three times asks Peter if he loves him and Peter three times affi rms his love. Three times, with varying words, Jesus charges Peter to exercise his charge in caring for Jesus ’ sheep. The sheep belong to Jesus, the true shepherd of the sheep, but Peter is charged with the responsibility of care for them. The perspective of John 21 presupposes the absence of Jesus so that Peter cares for his fl ock in his place.

• (5) The conclusion of John 21 also assumes the absence of the Beloved Disciple because, although the gospel is attributed to him, this conclusion is given in the voices of those who attest the veracity of what he wrote. What makes this necessary is the absence of the Beloved Disciple. The terms in which Peter discusses his fate with Jesus (John 21:20 – 3) suggest that the Beloved Disciple had died before the gospel was published with John 21. John 21:20 – 4 suggests the evangelist lived until old age. Throughout his long life he refl ected on the gospel tradition and developed his teaching. This process began in Judea but continued, after the Jewish War, somewhere in the Diaspora, perhaps in the vicinity of Ephesus. The tradition, even in the developed form we fi nd in the gospel, manifests its Judean origin. But the Greek of the gospel is a consequence of its Diaspora context. The evangelist refl ected on the gospel tradition and interpreted it to speak to the various changing situations until his death. The gospel makes the claim that the risen Jesus continues to speak through the inspired witness of those who knew him and whom the Spirit of Truth led into all truth. Even when the Beloved Disciple died, those who had been taught by him continued his work, as John 21 confi rms. We may only guess what additions were made to John 1 – 20. It may be that they added the references to their teacher as the Beloved Disciple and the explanations of Jewish festivals and terms.