EVE Online's prologue uses this trope, and has some clever ways to make it a bit more plausible. It's all laid out in their old intro video. Here's the summary...
Humanity has "outgrown Earth" and go out in a "desperate quest to colonize other worlds". Humanity builds FTL stargates to jump between worlds. Eventually even the stargates could not take them further (this could mean humanity covers the whole Milky Way galaxy). With nowhere to expand to, humanity begins fighting itself. Your basic "Earth Is Doomed" story, but it's the whole galaxy.
Then Earth discovers the "EVE Gate", a wormhole to another part of the universe called "New Eden". It's so far away they're not sure where it is in relation to Earth. Millions of colonists rush through and eventually settle thousands of new worlds.
Then one day, without warning, the gate collapses leaving thousands of colonies suddenly cut off from Earth and their supply line to Earth.
Lacking the ability to build, maintain, and repair space ships and jump gates without support from the Milky Way, each colony lost contact with the others for thousands of years. Most colonies are not yet self-sustaining nor are their planets even fully terraformed. Cut off from each other, these colonies die. The surviving isolated and incomplete colonies descend into anarchy and barbarism. This begins "a dark age that erases civilization as it is known to be, as the accumulated knowledge of millenia slowly eroded".
Finally two planets independently reinvent space and FTL travel, and humanity begins building stellar empires again. But the dark age has left "the memories of our past transformed into legend and myth".
EVE Online solves the problems with the "Earth That Was" in a few clever ways.
First, New Eden is so far away from Earth that even "the ancients" didn't know where it was in relation to Earth. One explanation is it is within the same Universe, but outside the universe observable from Earth. This neatly solves the problem of how they lost knowledge of where Earth is: they didn't know it in the first place.
Second, the only way to get to New Eden is via the EVE Gate wormhole. This leaves no breadcrumbs to follow back to Earth. There's no metaphorical "road" nor string of colonies nor evidence of space campsites to follow back to Earth as we can with, for example, Polynesian expansion. Even if they wanted to, the residents of New Eden can't trace the steps of the ancients. The ancients can't do it either.
Third, FTL in New Eden is limited. They model distances in game between planets and star systems accurately. The FTL drive in ships is sufficient to quickly move from planet to planet within a star system, they are far, far too slow to cover interstellar distances. For example, the distance from Earth to Saturn is about 1 billion km, give or take. The distance to Alpha Centauri, the nearest star system, is 25,000 billon km; 20,000 times further.
Most ships need established jump gates to travel between star systems. These represent an enormous investment in time and resources to establish and maintain, something struggling colonies could not afford. While there are ships with their own interstellar jump drives, they are enormous, expensive, fuel hungry, and limited in range. This makes casual interplanetary exploration difficult and expensive. Getting to the neighboring star system requires either an established jump gate, or an enormous investment in a jump capable capital ship, or a lot of patience. This reduces the ability for humanity to explore making it more plausible they can't find Earth again. Firefly does something similar with "The Verse" being just one very large and heavily modified star system.
And just to cover any holes, the "dark age" of New Eden is so much worse and complete and sudden than anything humanity has ever seen. In examples from Earth history, there's always some way communications or trade can happen, even if it's across a perilous ocean or desert. In New Eden, the loss of space flight leaves all the colonies cut off from each other by the cold vacuum of space. On top of that, most colonies die off, only a handful survive. Those survivors cannot communicate with other survivors for millennia.
This hard information and trade gap between colonies, plus the swiftness and suddenness of the EVE Gate's collapse, makes the New Eden dark age lengthy and complete. It is far more plausible that all history and technology would be wiped out with a millennia of isolation in marginal environments.
This long, complete dark age also allows EVE Online to reset the timeline of humanity. EVE Online is set millennia in the future. On this scale, humanity and its technology would be incomprehensible to a 21st century observer. It would be difficult to explain why we have the Space Spanish Inquisition fighting the Space French.
Instead, the New Eden dark age allows them to be divergent cultures emerging from their own isolated post-apocalyptic planets. Their sometimes primitive and regressive cultures, for example there is slavery in New Eden, can be explained because each one is the result of humanity being knocked back to basic survival in isolation. Each in their own Darwinian struggle on their own little Galápagos.
The closest analogy to New Eden from Earth's history might be Easter Island. Settled around 700 AD, geographically distant from any other inhabited island, it became completely isolated around 1500 AD. By the time Europeans arrived in 1722 it had consumed most of its resources and lost 80% of its population.