Settings where the world is a pale, degraded shadow of what came before? Arda, compared to Arda Unmarred; the world of Pact, compared to what a character theorises about a world before demons.
A related trope that's not this is general post-apocalypse and fallen civilisations.
I think part of this trope is the world before ran on _different rules_ that were grander.
@TetraspaceGrouping Oooh this is in Remembrance of Earth's Past as well! (the "Dark Forest" trilogy by Cixin Liu).
Spoilers
@TetraspaceGrouping At some point we see a spaceship come across a surviving pocket of four-dimensional space.
The lightspeed thing is because lightspeed (but not FTL) drives are possible, but they permanently lower the lightspeed in that area - advanced civs have gradually driven lightspeed across the universe down to the current value because they're all using lightspeed drives constantly.
Spoilers
@TetraspaceGrouping This is also stated to be why string theory seems to suggest the universe has a bunch of extra dimensions, but some of them are randomly super compact and impossible to observe - it's not "randomly", it's because those dimensions were collapsed!
Spoilers
@TetraspaceGrouping Basically the universe used to have more spatial and temporal dimensions, a higher speed of light, and probably some other stuff. Advanced civs developed weapons that collapsed some of these dimensions locally, and protect themselves by restructuring their own civilization to work in lower dimensions. Eventually our own solar system is wiped out by a 2-dimensionifying weapon.