On the FRDB forum, Jake Jones IV indicated that not only Marcion's version of 'Romans' did not include chapters 15 & 16, but also many copies as witnessed from the end of the 2nd century up to the 5th century.
I had to check the evidence.
For Marcion, the testimony of Origen is very clear:
Marcion, by whom the evangelical and apostolic writings were falsified, removed
this section [16:25-27 the doxology] completely from the epistle,
only so, but deleted everything from that place where it is written, whatsoever
is not of faith is sin, [14:23] right to the end. Origen, Commentary on the epistle to the Romans, XIV, 1290 AB
Also, for the five main commentators after Marcion (Irenaeus, Clement of Alexandria, Tertullian, Origen & Cyprian --180 to 250), in their known works, Clement witnessed all sixteen chapters, Origen all of them except ch. 4, Irenaeus & Tertullian, all except ch. 15 & 16 and Cyprian was silent on ch. 4, 6, 7, 10, 15 & 16.
Tertullian indicated, concerning 'Romans' and Marcion's version:
"But what serious gaps Marcion has made in this epistle especially, by withdrawing whole passages at his will, will be clear from the unmedullated text of our own copy. It is enough for my purpose to accept in evidence of its truth what he has seen fit to leave unerased" (AM, book V, chapter XIII)
However Tertullian did not tell which passages were deleted in Marcion's version.
Also 'the Concordia epistularum Pauli' (a guide to themes in the letters cross-referenced to the Amiatine chapters) and three ancient manuscripts do not have anything about chapters 15 & 16 (Ref: here).
Therefore, it is undeniable there were "orthodox" copies of 'Romans' after Marcion's times which did not have chapters 15 & 16.
And why did orthodox Christians, after Marcion's times, avoid including chapters 15 & 16, which were existing then (earliest reference: Clement of Alexandria, around 200 CE) and adopt heretic Marcion's short ending of 'Romans'? A reasonable explanation would be these copyists used older "orthodox" manuscripts of 'Romans' with the short ending.
Now, let's examine the following hypotheses:
a) Did Marcion "fabricate" 'Romans' and "orthodox" Christians added up later ch. 15 & 16?
b) Did Marcion work from a copy of 'Romans' with ch. 15 & 16 missing?
First, let's start by a minor argument:
Marcion had no reason to cut off all of ch. 15 & 16, even if parts of them were against his views (such as the scripture quotes, Jesus having been a minister to the Jews, the trip to Jerusalem in order to bring money to the (Jewish) "saints"). The following passages would have been acceptable for Marcion: 15:1-7, 13-20, 22-24 & 16:1-23.
Second, no other Pauline epistle ends so abruptly, that is with no conclusion or/and greetings. After such a long epistle, and a lengthy introduction, an absence of proper ending is inexplicable, except, for unknown reasons, it was missing in copies early on (and therefore not available to Marcion and others after him). Certainly 14:22-23 looks to be a conclusion, but only to chapter 14, against vegetarianism but in favor of tolerance. That conclusion is not for the whole letter.
Furthermore, from my perspective, chapters 15 & 16 (up to verse 24) are very much authentically Pauline, with typical scripture quotes, allusions to an earthly & human Jesus (15:8, 12), traveling plan, the collection for the "saints" of Jerusalem (as in 2 Corinthians 8 & 9) and greetings. And I do not see any reason why they would not be the missing parts that Marcion and possibly Irenaeus, Tertullian & Cyprian did not have (but others, like p46 --an early manuscript dated around 200--, Origen & Clement of Alexandria had).
I do have evidence to justify this from '1 Clement' (see here for dating).
The author of '1 Clement' (writing from Rome) did not have Peter & Paul visiting Rome, nor going through suffering deaths, but apparently thought that Paul went to the western end of the earth (like Spain as believed in these days):
5:4 There was Peter who by reason of unrighteous jealousy endured not one
not one but many labors, and thus having borne his testimony went to
his appointed place of glory.
5:5 By reason of jealousy and strife Paul by his example pointed out the
prize of patient endurance. After that he had been seven times in
bonds, had been driven into exile, had been stoned, had preached in
the East and in the West, he won the noble renown which was the
reward of his faith,
5:6 having taught righteousness unto the whole world and having reached
the farthest bounds of the West; ...
Then, from where did "Clement" get his "information"? Very likely from chapter 15 of 'Romans':
15:23-24a But now that there is no more place for me to work in these regions, and since I
have been longing for many years to visit you,
I plan to do so when I go to Spain. ...
15:28 So after I have completed this task and have made sure that they have received
this contribution, I will go to Spain and visit you on the way.
Note: it appears "Clement" plagiarized part of Ro 1:32 (1 Cle 35:6), 9:5 (1 Cle 32:2) & 12:5 (1 Cle 46:7), and of course he knew about at least one of the Corinthians letters (1 Cle 47:1-4).
The doxology (Ro 16:25-27) is rather pompous, consisting of a very long sentence, with expression like "according to ... the preaching of Jesus Christ" (never used by Paul before, in any variations) and obviously not written by Paul. It might have been generated in order to provide a suitable conclusion for the copies ending (abruptly) at 14:23 (note: Three ancient vulgate manuscripts omit chapter 15 & 16 but have the doxology).
But it was also added to the longer version of Romans (after 16:23). Actually, that doxology (in the form of a grandiose conclusion) appeared in different places in the oldest known manuscripts (or witnessed by ancient writers), such as:
After 14:23, 25 times; after 16:23, 20 times; after both 14:23 & 16:23, 8 times; after 15:33, once (in p46).
Also, for about 13 times there is no doxology.
What to conclude from all that? Early on (before Marcion), 'Romans', with chapters 15 & 16 missing, was used to make copies. And after Marcion's time, among orthodox Christians, for some time, those co-existed with copies of the full versions.
a) 'Romans' could not end abruptly at 14:23, with no general conclusion, no greetings & no doxology.
b) Chapters 15 & 16 (except the doxology) are typically Pauline & fit well at the end of the letter. Furthermore '1 Clement' surmises Paul went to the extremities of the West, as Paul planned to do in ch. 15 (going to Spain).
The complete letter (but without 16:25-27) had to be written first, before, for unknown reason, copies were made without the two last chapters (with later the doxology being added at the end of ch. 14 in order to provide a badly needed conclusion).
Therefore 'Romans' had to exist in its entirety (up to 16:23) before Marcion's version was written.
Note: Richard Carrier in OHJ ("On The Historicity of Jesus") thinks the doxology was written by Paul (pages 48, 135, 137-138, 140, 517) and therefore is not an interpolation.
All along his book, Carrier uses Romans 16:25-27 (the doxology) abundantly (about seventeen times) in favor of his agenda, as on page 594: "Romans 16.25-26 outright says the 'gospel' and 'preaching' of Jesus Christ was discovered by revelation and finding secrets hidden in the scripture. We should conclude that's indeed exactly what happened."
Here is Romans 16:25-26 (RSV): "Now to him who is able to strengthen you according to my gospel and the preaching of Jesus Christ, according to the revelation of the mystery which was kept secret for long ages
but is now disclosed and through the prophetic writings is made known to all nations, according to the command of the eternal God, to bring about the obedience of faith--"
Remark: the Greek root word for "preaching" is not used anywhere else in Romans. It can mean "proclamation" or "heralding" rather than "preaching". Therefore "of" meaning "about" instead of "by" makes more sense and would destroy (with Romans 16:25-27 being a late interpolation) Carrier's argument. And, of course, "according to" does not have to mean Paul's gospel and the "preaching" of Jesus was copied from the scriptures, but only was in accordance with them (or rather their Christian interpretation!).