I would have thought that it was quite the opposite: you underline or write in books you intend to keep and go back to. The notations are for the benefit of your future self.
Me too. I was brought up in a literate but not affluent family. It would have been considered Very Bad Form to write in library books.
My dad’s scriptures have a lot of underlining in them from when he was a missionary, so Church of Jesus Christ of Latter Day Saints was doing this at least in the 1960’s (though as another thread mentions, it’s not like people think of us as Christians, and Evangelical Christians definitely don’t, and in any case whatever you think of our theology our culture will be somewhat different).
I have underlined and scribbled notes on various things in my scriptures too (mostly in the 90’s), and it’s always neat when I am going back and looking at them to see a note in it where I’m like “oh, right, I remember that interesting interpretation we talked about in college!” or whatever.
I do not write in any book but the scriptures, and I don’t write in any of my more disposable/cheap Bibles etc.; the one I write in is the leather-bound gilded-pages inscribed-with-my-name edition that I have had since my parents gave to me when I was in high school because I do want to keep these notes forever.
That’s pretty much what I was thinking; the answer to the OP is when Bibles went from being expensive, treasured family heirlooms, to mass-produced books that people could afford to get several of, without any real expectation of bequeathing them to anyone.
My dad refused to go to the church my mom went to, because one day in Sunday School, my dad made a comment about some point of theology, and used chapter & verse to illustrate it, and some other guy said “Well MY bible says…” and proceeded to nitpick dad’s comment based on the way that his KJV differed from dad’s NIV (or whatever non 17th century translation it was). From what dad said, he basically Peace-Outed out of the class right then and there, because of the ridiculousness of the argument and the earnest and serious way everyone else treated it.
Since there were stories that a pocket Bible (as well as cigar cases, hip flasks etc) stopped a bullet, I’d be wanting my family to send over the jumbo family-size easy-reader large type coffee table edition, thank you. 6 x 4 seems far too small to trust a heathen enemy to aim at accurately.
Smithsonian magazine story - American Civil War
Daily Mail - WW1
I would assume it’s always been a thing. I don’t know that for sure, but writing in bibles has been common practice for hundreds of years – people would write records of their families (births, marriages, deaths, etc) in the front or the back, and those are used in genealogy research.
I’m not currently any sort of practicing Christian, but I grew up amongst the bible-thumpers, was gifted a bible, and attended the sort of Sunday School classes where they didn’t know what to make of my questions. I wrote notes in that bible, and no one had issues with that (although I don’t think they would like the notes I made, if they read them).
Thomas Jefferson famously edited his bible into something more agreeable to his Deist religious views: Jefferson Bible
Obviously, he wasn’t the Evangelical type, but it’s more evidence that “not writing in bibles” has not really been a big element of bible-having.
I owe my degree to one used “math” tome that the previous to previous owner had put All the answer in very easy to follow margin notes…hey, you do what you gotta do.
And yet, when I looked at history, poetry, or economics in the library, that’s what I found. Not so much in physical sciences.
There are a lot of Christians who study the Bible just because they like to study the Bible. They aren’t looking to evangelize or proselytize, but they’re really interested in why Matthew’s version of X differed from Luke’s, or why John included this story but not that one. Just like some people who aren’t currently majoring in classical English literature study Shakespeare.
And they all love to mark up their books.
My father was giving away pocket-sized Gideon’s New Testament bibles to school students well into the 1990’s. He was the local rep for Gideons, so we always had boxes of fresh bibles in storage.
I was raised with too much respect for books of all types to annotate my own bibles - such notes are often for the moment, and you end up losing all those reminders when you upgrade or lose/donate it. I never saw the point.
These days, annotations/notes on a Kobo edition bible or in a mobile app are much more my style.
I grew up in a household where books - ALL books - were sacred objects and never to be written in (except workbooks and coloring books, of course. And puzzle books. Just those).
Entering college it was a real mindscrew for me to not only experience people who marked up books but to learn it was expected of me to do the same. I sort of got past it, but always felt I was doing something wrong when I did it.
One thing about e-books is that mark-ups in them don’t permanently mar the book.
I realize, of course, that many, many people have a different approach.
Unfortunately, according to the bible itself, scrawling the proof would be a sin.
Revelation 22:18-19 (King James Version):
… From which I take to mean Jehova is not a god of copy-editors.
Wow! Scribble in my book and I’ll give you lethal buboes. Escalatory.
You know, that book (Revelation) is all the way at the end, too – what if you were marking up as you went along, only to read that after you are almost done? Diabolical!
Literally!
That’s why many old translations call it the Book of the Apocalypse
Speaking of Bible “marginalia”, I once read somewhere that women of easy virtue wrote their contact information inside Gideon Bibles in motel rooms.
For awhile there I checked the Gideon Bible whenever I stayed in a motel (out of purely academic interest, I assure you). Never found any hooker marginalia. 
To me the passage pretty clearly means do not add your own extra content to the text of the book (and why would it not just refer to this book, when at the time the entirety of Christian Scriptures was still not canonically compiled.)
So, “think about this!” on the margin, or a little smiley face heart next to where Jesus blesses the children, OK.
New End Times prophecies, or fanfic about characters, go write your own bestselling novel.
Which just means “Revelation”. Modern English took off from the content about End Times prophecy and adopted using the word to mean a terminal calamity.
See Deuteronomy 4:2, which has a similar warning. By the logic that purports that no scripture can come after the book of Revelation, one would have to exclude everything after Deuteronomy.