The spacetime horizon you’re most likely familiar with is the “event horizon” of a black hole, forming the “surface”. The event horizon is not just a boundary in space, it is in some sense a boundary in time. Someone falling into a black hole shouldn’t notice anything too exciting at the event horizon. At the event horizon of a black hole this slowing effect reaches it’s natural extreme, and time stops. Just like the horizon on Earth, the cosmic event horizon is a regular place.