It’s the universal speed limit for material objects; spacetime isn’t a material object.
It’s not a speed limit as such. You don’t go up to it and then find you can’t go any faster. It’s more like a horizon that you can never reach. Even in a flat spacetime model there’s plenty of “warping” in the Lorentz Transformation.
I recall that in FARMER IN THE SKY, I believe it was, the young hero questions the spaceship engineer what happens if you reach “almost” the speed of light and then go full blast. In the story, the engineer has no answer, only proving the author’s limited grasp of this subject.