First of all, I've browsed all FAQ and haven't found this trope. I'm also non-native, so some mistakes may appear. I hope I'm at least understandable.
From what I know several decades after their death historians start to analyze historic figures correspondency. They analyze letters and journals and take a lot of information from it.
But world has changed recently and we shifted to Messenger, iMessage, WhatsApp and others. There are still tons of hobbyists who write letters to each other, but it became a choice, not main method of communication.
So imagine if some important historic figure died. 50 years later we want to analyze their life. As our imaginary figure prefered to not write letters, modern communicators and e-mails are the only way to go. They are bustling with their private conversations, arguments, letters, dramas, voice messages... Will historians be able to check it? Are there currently any debates about this problem that will grow really large in upcoming decades?
(Personally I can imagine big corporations setting up some "legacy share" option for researchers to ask about certain archives of historic figures' private convos. But maybe I'm wrong)
I'm going to answer this as a librarian and archivist, because we're the folks who actively preserve the information for historians at the time of its creation. This may be the single biggest question facing these professions today. The short answer is that yes, there are huge debates and discussions within the field.
This is the era of "born digital" information, where everything is documented digitally, and very little if any of the most important parts of our lives are committed to paper. Data is a LOT more fragile than paper, so there are any number of concerns surrounding this seachange.
One of the major issues is simply the sheer number of for-profit communication tools. Email, text, and other messaging programs (not to mention Reddit comments or Tumblr blogs) would all be great sources for writing biographies. Connecting all of these to the same person, though, is a major challenge. What might be a collection of letters and some commonplace books (where interesting images or articles are glued or copied in) is now scattered across multiple platforms.
Each of these platforms is a for-profit corporation. When Yahoo shut down Geocities, hundreds of terabytes of data were lost--mostly what we might call low-importance websites, with all sorts of silly flashing gifs and garbage webdesign. However, taken together, they could have been (and could still be; there was a distributed effort at downloading and archiving at least part of Geocities) an incredibly important source documenting all sorts of ordinary peoples' reactions to the advent of the World Wide Web. Similarly, Myspace has deleted all sorts of data as it has hemorrhaged users. Any storage medium where access to the data is mediated by a for-profit corporation or an organization not explicitly committed to long-term preservation must be considered temporary.
In order to protect this data for the future, it's vital that it be stored in a way that is intentionally archival: all of this data has to be stored by organizations which are committed to longterm safekeeping of this data. This is its own issue--data storage is a lot more expensive than putting a box of letters on a shelf.
The next big issue is how to do that data storage safely.
Say the Clinton Administration had a really important document in 1995. They write it in in WordPerfect, and save it on a floppy disk. Fast forward to 2020. Well, first of all, even finding a floppy disk drive is a challenge--we'll need a USB floppy drive, which isn't common or cheap. Even once you get the drive talking to a computer, well, floppies are magnetic media and the magnetization will fade without use over time. If no one has opened that disk since 1995, the data is probably gone. Even passing through those hurdles, how are we going to open that WordPerfect file? A third-party conversion software is needed to get that information out.
We don't know what current file formats are going to be the WordPerfect of the future, but we know that it's already increasingly difficult to read data from HD-DVDs. Large research libraries buy up obsolete readers and drives for all sorts of media, from the common to the obscure, because you never know what will show up stored on what, and try to maintain obsolete computers because of the programs they have installed.
There are archival standards for longterm, lossless storage of materials. The most common concerns are text documents and still images, the standard for which are PDF/A and TIFF files, respectively.
Let's assume that we have migrated the Clinton Administration data into PDF/A format: we got it out of WordPerfect. Now what? What media do we put it on for long-term storage?
The answer is that there isn't an answer. No media has a dependable tested lifespan of longer than 20 years (though I will say that the testing constraints for magnetic tape are a little odd). Constant migration and refreshing of data is an important part of data preservation: we can't copy that Clinton Administration document to a hard drive and forget it on a shelf for thirty years.
The other method of preservation is distributed storage, both in terms of types of storage media and its location. This is something that's often described as the LOCKSS method of preservation: Lots Of Copies Keep Stuff Safe. For any digital item, there is typically a "master" copy and at least one, often many, "access copies". The master copy is kept in a very stable but slow-to-access format (perhaps a tape drive in a vault). The access copy is kept onsite, on a flash drive or a spinning disk, and is used for researcher access. The master copy is only accessed to make additional access copies, and is regularly copied onto new, up-to-date media.
All this is the ideal, and expensive. As any historian knows, there are LOTS of lacunae in the historical record. There will be more in the digital age: there's no way for a shepherd boy to stumble into a cave and discover 2000-year-old laptops with readable data. Anything we want to keep we will have to actively work to keep.
Sources:
To Preserve and Protect: Strategic Stewardship of Cultural Resources. Library of Congress, 2002
The Information, Gleick, 2011
Preserving Archives and Manuscripts. Second edition. Ritzenthaler, 2010.
Developing and Maintaining Practical Archives. Second Edition. Hunter, 2003.
The relevant FAQ section is difficult to find. /u/caffarelli writes a lot about being an archivist on this subreddit. But just because someone else has asked a version of this question before doesn't mean it can't be asked again.