Minecraft wiki entry for World_boundary, associated text: Attempting to walk into the boundary will still display a walking animation, walking sounds, and cause view bobbing, effects which are not present for walking into the world border or solid walls.[1]
Using elytra to fly into the border will cause the flying sound to continue to play at full volume rather than quietly or not at all.[2]
Rain particles will always hit the ground at exactly sea level regardless of the actual shape of terrain beyond 30 million blocks.[3]