So my advice is to learn about manuscripts.
Praise the LORD!
The NIV and NWT follow the same pattern:
Mark 16:9-20
https://www.biblegateway.com/passage/?search=Mark+16:9&version=NIV
https://www.jw.org/en/library/bible/study-bible/books/mark/16/#v41016008
The words do not occur in the following corrupted texts:
Aleph (Sinaiticus) [which omits large portions of the OT & adds Shep. of Hermes, & Ep. Barnabus], B (Vaticanus), pc (N/A, UBS cite 1 cursive, and say that some others make reference to a shorter ending), (k), sin, Coptic: sa-ms, Arm-mss.
Here is the vast extant literature which has the text in it:
"... A, C, D, E, F, G, (H), K, (L), M, S, U, V, (W), X, Y
Gamma, Delta, Theta, Pi, Sigma, (Psi), Omega
(099), (0112)
Cursives: MAJORITY, fam 1,13
Old Latin: aur, c, d-suppl, ff2, g1,2, l, n, o, q, r2, Vulgate
Syriac: Pes.hitta, Curetonian, Harclean, Palestinian,
Gothic, Armenian, Ethiopic
Also extant in 047,. 055, 0211, 0233?, 0257. Early bishops who claimed to be direct successors of the apostles would find their inability to perform the works of the apostles (II Cor. 12:12) a matter of embarrassment. Are we really to believe that the Gospel of Mark would end in verse 8 with the words, "for they were afraid"? See Burgons great work on this chapter. ..." - A Closer Look: Early Manuscripts & The A.V.; by Jack Moorman, page 85
Additionally:
"... TATIAN, Diatessaron, (I 10:125,126,128,129), in entirety.
IRENAEUS, Against Heresies (I 1:426), vs. 16:18
TERTULLIAN, Resurrection of Flesh (I 3:584), vs. 16:19; Against Praxeas (I 3:627), vs. 16:19
CYPRIAN, 7th Council (I 5:569), refers to vs. 16:17,18; Gospel of Nicodemus (I 8:422,432,436,445), vs. 16:15-19
APHRAHAT, Demonstrations (III 13:351), vs. 16:16-19
APOSTOLIC CONST. (I 7:457,479), vs 16:16-19
AMBROSE, The Holy Spirit (III 10:133,134), vs. 16:15-18; Const. of Faith (III 10:216), vs. 16:15; Repentance (III 10:335), vs. 16:17,18.
Nothing in indexes of the ANPF of pre-400 AD father quoting up to vs. 8 and stopping. ..." - Early Church Fathers and the Authorized Version, by Jack Moorman, page 44
Additionally:
"... The evidence in favour of the authenticity of Mark 16:9-20 is overwhelming. The TBS publication (58) "The Authenticity of The Last Twelve Verses of...Mark" is an excellent summary, drawing mainly from Burgon, (14) p 36-40, 422-4 and Burgon's work cited by Fuller (33) p 25-130. See also Burton (5) p 62-3, Fuller (4) p 168-9, Hills (3) p 161-2, (38) p 133-4, Ruckman (2) p 132.
The TBS publication-see above-states that only 2 Greek manuscripts (Aleph and B) out of a total of 620 which contain the Gospel of Mark, omit the verses. See Burgon, cited by Fuller (33) p 60-1. Moreover, Burgon, ibid p 67, states that a blank space has been left in B, where the verses should have been but where the scribe obviously omitted them.
As further evidence in favour of the verses, Burgon (14) p423, (3) p 169, cites: 2nd Century: Old Latin and Peshitta Syriac versions, Papias, Justin Martyr, Irenaeus, Tertullian; 3rd Century: Coptic and Sahidic versions, Hippolytus, Vincentius, 'Acta Pilati'-by an unknown author, Apostolic Constitutions; 4th Century: Curetonian Syriac and Gothic versions, Syriac table of Canons, Eusebius, Macarius Magnes, Aphraates, Didymus, The Syriac "Acts of the Apostles," Epiphanius, Leontius, Ephraem, Ambrose, Chrysostom, Jerome, Augustine; 5th Century: Armenian version (some copies), Codices A and , Leo, Nestorius, Cyril of Alexandria, Victor of Antioch, Patricius, Marius Mercator; 6th and 7th Centuries: Codex D, Georgian and Ethiopic versions, Hesychius, Gregentius, Prosper, Archbishop John of Thessalonica, Bishop Modestus of Jerusalem.
The TBS also cites the Philoxenian Syriac of the 5th century as containing the verses. Hills and Ruckman also cite Tatian (2nd century) as quoting the verses. Hills (3) p 162, (38) p 134, states that besides Aleph and B, the Sinaitic Syriac-from the same source as Aleph, 2 manuscripts of the Georgian version and 62 of the Armenian version omit the verses. The Old Latin manuscript k has the "short conclusion" instead of verses 9-20. See notes for NEB, NWT. Burgon (33) p 81-2, explains how this short ending has been obtained solely from Codex L, an 8th or 9th century manuscript "with an exceedingly vicious text" (ibid). Hills explains the omission of verses 9-20 from the above handful of documents as indicative of the work of heretics, especially docetists who sought to de-emphasise post resurrection appearances of the Lord from the Gospel record, ibid p 166-8, p 138-41.
Burgon (33) p 49-60 also demonstrated that the supposed adverse testimony of ancient writers is spurious, resting on a quotation from Eusebius which does NOT deny verses 9-20. Berry's Greek text supports this passage.
Let's take a look at the two "oldest and best" manuscripts that delete the last twelve verses of Mark 16. The Vaticanus (Codex B) and Sinaiticus (Codex Aleph):
The Vatican copy stops short at the end of Mark 16, verse eight. But the copiest left a blank space sufficient to accommodate the twelve missing verses! This is the only vacant column in the whole Vaticanus manuscript! It seems that the copyist knew that there was a portion missing in the copy before him. Dean John William Burgon draws the obvious conclusion that the scribe who prepared Vaticanus "was instructed to leave them out, and he obeyed; but he prudently left a blank space in memoriam rei. Never was blank more intelligible! Never was silence more eloquent!" (op. cit., p. 67, "Last Twelve Verses of St. Mark," 1871).
As for the Sinaiticus manuscript, it is written in the same-size letters throughout until you come to the place where the last twelve verses of Mark belong, then the letters become large and spread out, taking up enough extra space to allow the last twelve verses of Mark to appear in the smaller letters that had been used up until this time. The double page containing the end of Mark and the beginning of Luke was removed at an early date and replaced with the four sides rewritten to exclude Mark 16:9-21! By slightly increasing the size of the letters and spaces, the writer was able to extend his shortened version to the top of the column preceding Luke one. Tischendorf, the discoverer of the Sinaiticus copy, alleged that these pages were written by the copyists of the Vaticanus manuscript.
Additionally:
"... Main line Bibles always had it: AV (KJB), HF & CR*; Tyndale, Great, Geneva, Bishops, Stephanus, Th. Beza, Elzivir bro., E, 238, 239, 440, 475-2nd. cor., 477, 486, 488, 489, 1071, & etc. ..." -
https://brandplucked.webs.com/mark16920.htm
Basically in 10,000's of MSS.