Is everything posted online? I would assume. It might not be in one place, though. People have the compulsive need to flap their lips. Even regarding really sensitive information. I find it hard to believe there’s information out there that has not been posted online.
But, I’m just an average Joe, what do I know?
Is a lot of information still in books? Passed down from competent mentors?
Not all old printed material has been digitized, nor even will all of it ever be. And even if when it is, it’s often just in low-quality scans that can’t be searched. And a lot of what’s passed down from mentor to apprentice can’t really be condensed into a static form.
Plus, of course, there’s plenty of information which has been lost completely, and is no longer available in any form. And far more information which nobody has ever had yet.
The contents of my wallet, safe deposit box, and any ammunition storage containers I may or may not own are not now and never will be online. So no, all information is not available online.
Here’s an example from the locksmithing world. In the 1970s and 80s, vehicle manufacturers kept databases correlating key codes with Vehicle ID Numbers. They made copies of this database on microfiche and handed them out to all the dealerships. If you ever lost your car key, you could ask the dealership to look up your key code, take that information to a locksmith, and get a new key. Around 1990 or so, they started putting the database in electronic format. But they never bothered to go back to the prior years and transfer the information from the microfiche to the computer. Eventually, all the dealerships just tossed the microfiche into the dumpster and now that information is lost forever. If your car is a 1996 there’s a very good chance the dealer can look up your VIN in their computer and find the key code. But for a 1986 the chances are virtually zero.
There was an episode of Law & Order where a young lawyer complained about having to search, by hand, printed law books published prior to 1990. The second lawyer commented that no one wanted the job of taking all that old information and typing it into a computer.
Off the top of my head, I can think of lots of things that never get recorded anywhere. What color is my windbreaker? That’s not a secret or anything. You can just look at me and see what the color is. Sure, the store I bought it from made a record of the original purchase, but there was no record made when it was donated to the thrift store and my receipt from when I purchased it at the thrift store only record the fact that it was menswear, didn’t even record the type of clothing item, let alone the color. Hmm OTOH, maybe you could find a picture of me on Facebook where I’m wearing it. Perhaps that’s not a perfect example after all.
That’s for me to know and you to find out. 
Nope. I just did a historical talk, which included stuff from papers in a conference back to 1984. None of this exists online - IEEE didn’t start digitizing them until a bit later. Luckily I had copies in my garage.
I suspect a good number of my 1950s SF magazines are not online either.
There’s vast amounts of information that isn’t online, but even more information that is on-line but nearly impossible to retrieve, because it is so poorly indexed.
My Google-fu is off today. I can’t find a cite now, but I remember reading that more words and numbers were published online between 1995 and 2007 than all the words and numbers ever published on paper prior to 1995. Forgive me if I got the years wrong; I’m going from memory. In just over a decade, we doubled the amount of information the human species has recorded. Then between 2007 and 2016 we doubled it again. So, for things like books, scientific papers, news articles, blog posts, letters, emails, etc, you could make the case that 75% of all the information that humans have ever created is in digital form, precisely because it was created after 1995. Following this trend into the future, I can easily imagine 85%, 90%, 95%, etc. But that will never cover things prior to 1990-ish or things that nobody bothers to record, like what I ate for dinner last night.
And of that information which is online and in a retrievable form, a lot of it is on secured sites online, accessible only to people with the proper authorization (such as paid subscribers).
I am always amazed how unreliable a record the web is in many areas. I regularly look up something I remember from a book I have read (typically historical or scientific trivia), but can find no record of it online, and have to go back to the book I read it from to find a cite.
Would be hard to google most of the things in the Vatican Secret Archives.
Yes, it is often a source of frustration to me that Google stopped supplying newspaper archives and now you need a subscription to newspapers.com.
Welcome to Geneology 101. :rolleyes:
I searched for “genealogy” but it didn’t find your post!
Someone over at the NSA is chuckling right now. 
We’re in the process of cleaning out my house which contains stuff which originally belonged to my great-great grandparents. I’ve found drawings my mother made as a kid in the 1930’s, the notes my dad made in his US sailor hard hat diving classes in the 1950’s, and my own early attempts to create comic books in the 1960’s. I guarantee none of that stuff is anywhere online, nor will it ever be.
My home is chock-full of information, and the majority of it can not be found online. I suspect it’s that way for most people. I also suspect it’ll stay that way for a long while yet; I have no plans on posting regular updates on how many cans of chicken consomme I have in the pantry, nor inventory the bin full of recyclables.
As a tax researcher, I may in theory want to go back and look at the legislative history of a law to see what the intent of Congress was. If it passed relatively recently, there’s a good chance that all the hearing about the law are digitized and available somewhere. But from the pre-digital era? The library where I went to school is apparently one that is frequented by people wanting to track down this stuff because they have some stuff that no one else in the area has. It’s such a niche use that no one really wants to take the time to digitize it all. There are other tax things that are unlikely to be found online as well due to age, but those legislative histories are the least likely to be easily searchable.
I’m no expert on information theory, but I imagine that it’s totally impossible, even in theory, to publish all information on-line. I suspect Kurt Gödel, or at least Doug Hofstadter or maybe Russell, may have had something to say about this.
What have I got in my pocket?
I doubt you will ever know the contents of the SDMB Mod Manual (aka, Hookers & Blow Guide to Chicago’s Best, Brightest, and Occasionally Tepid).