Have we discovered everything that we could have discovered about our history?

Not in the slightest, I would say. There is an isle of infinite complexity in every period of history where there is sufficient documentation. There are so many topics that one can explore throughout history and learn new things. That’s why there are ever expanding fields in historical study. From economics, sciences and medicine, through poetry writing and music, to language and sociology. There is a lot to examine and research and put in context so that you understand some aspect of a historical period, or a personality, or an event.

But also, history is an evolving field because time has a habit of passing and events eventually becoming historical, and left for research. So effectively, history has a tendency to grow and grow as a field of study.

Just a few days ago there was a question regarding how a prominent political party in a certain country lost its prominence and relevance. I don’t believe it was answered yet, but to my limited understanding, the topic is still at least in a large part involves events that break the 20 year rule. Now, in a few years that could be researched and explored and given a very comprehensive answer. And so any event or process of the recent past will eventually be a historical one and could possibly be explored.

This question comes up on this sub from time to time and I always find it amusing. That's no shade to you, OP, just me peaking my head out from books about civilizations most people haven't even heard of to snicker a little bit. Like the other answers said, the answer is "Of course not!" Obviously, we can never actually run out of history because today's current events are tomorrow's historical studies, but that's just piling new history on top of mountains of still-undiscovered old history that we all already have. I'm going to lean pretty heavily on ancient examples here because that's my specialty, but most of what I'm talking about can be extrapolated out to other periods too. I think these questions tend to come from a place of both misunderstanding what exactly historians and focusing on familiar history.

History is not just the study of listing out events that happened before right now, but the study of interpreting the sources we have for those events. As a field, we can literally never run out of material because all you have to do to find something new is sit down with what we already have and come up with a new way of thinking about it. u/Iphikrates has written extensively on here about how recent scholarship is completely reassessing how we approach Ancient Greek warfare, a topic that has been studied to death a thousand times over. He and I were just two of several contributors to this recent thread about what historians are currently working on.

But it's not all reassessing old evidence. There are tens of thousands, maybe millions, of clay tablets, papyrus leaves, scraps of parchment, and other bits of writing just sitting in storage right now. They've never been translated and transcribed. Some probably haven't been looked at in decades. These are mostly held by museums and universities, but occasionally something hangs around in a private collection for years without academia even knowing it exists. Are these likely to reveal new events that changed the course of political history for heavily studied topics like Rome or medieval Europe? Probably not, but for less studied regions like Central Asia or Bronze Age Syria, it's entirely possible. In my field, a collection of leather documents from ancient Bactria were only published from a private collection in 2012, suddenly revealing the presence of known historical figures like Alexander the Great's last Persian adversary, Bessus, engaged in military actions on the northern frontiers of the Persian Empire. Prior to that reveal, modern scholarship was totally reliant on assumptions and trying to interpret Persian buildings uncovered by archaeology in the region.

Then there's the things we know existed, but have never found. For example, Washukanni, the capital of the Mitanni Kingdom that rivaled Egypt and Assyria in the late Bronze Age, has never been discovered. The ruins must be out there somewhere, but we have no idea where. An entirely different, previously unknown Mitanni city was identified when drought depleted an Iraqi reservoir in 2018, and then only accessed again earlier this year (due to more drought unfortunately. These are hardly the only "lost" cities that were referenced in writing but never identified to modern archaeology, and any of those cities would potentially contain new sources of information. In other cases, we know exactly where artefacts are likely to be, but they're underneath modern cities, and can only be excavated when construction happens to turn them up.

History also includes the study of literature, science, economics, art, and basically everything else that humans might have done in the past. That's where these unknown or untranslated items really have a chance to shine. There might be religious invocations, drawings, or financial records in there, and every one of those items can help us understand what ideas and systems people in the past were using. In my field, the Persepolis Fortification Archive is a treasure trove of information spread out over more than ten thousand tablets. Even though the bulk of obviously interesting tablets were published in the 1930s, new papers on individual tablets come out every year, and sometimes they reveal very surprising in detail. In 2007, more than 70 years after the first publication, we discovered a single tablet written in Old Persian. Up to that point, Old Persian was only known from monumental inscriptions. This one tablet completely upended our understanding of how the language was used!

Then there's the stuff we have, but can't even read. There are whole undeciphered languages out there waiting for the code to be cracked and unveil whole civilizations' worth of writing and records that we know almost nothing about. I've written more about that here regarding the Proto-Elamite and Indus Valley scripts, but they're not the only ones.

From my own experience, there's problems with the tools we use to write history and when they get better it will change what we can say about the past. For example, im a medieval art historian and any catalogue of manuscripts held in a particular library or archive is usually lacking information about images and decoration (e.g., Ker's catalogue of manuscripts in British libraries, even some places still rely on E.R.James work which is a century old at this stage). Often I open a new manuscript and either there's more in there than I expected or the information in the catalogue turns out to be quite wrong. This goes the same for Latin editions of works from the period as well, where quite often the only edited version is from the Patrologia Latina which is over 150 years old and the editor Migné often didn't say where he got the bloody text from (it's infuriating). That's just my own work, I suspect every historian has similar problems no matter their speciality. I've been able to write a history of medieval architectural drawing from before the 13th century, which I suspect many people would not have thought possible even 40 or 30 years ago. Things change and we get better at being historians and we get better tools to do the job (either conceptual tools or digital ones)

There is an awful lot sitting in museums and archives that nobody has looked at since it was donated... sometimes centuries ago. Seriously! Some religious orders are not the most well-organised about their archival collections. Stuff that was tucked away during the early colonial period has not always even been indexed, let alone studied. Private collectors can hand down historical treasures for generations only for them to suddenly show up at an auction. These things were once, perhaps, "discovered", but they have to be discovered again if we're going to learn anything from them.