The main assumption here is that protons are the only debris in interstellar space that might be a danger for starships travelling at relativistic speeds. The impact of a proton would create a shower of particles passing through the starship and destroying everything inside & out.

So I propose a type of shielding called a Relativistic Nuclear Diffraction Shield:

Consisting of a single layer of antiprotons arrayed perpendicular to the direction of travel, only a small amount of energy would be required to nudge an incoming proton sideways a proton-diameter or two so that it passes between the antiprotons that make up the shield, rather than collide with them. What results is an inelastic collision...

The incoming proton reacts via the weak nuclear force, converting its kinetic energy into mass and abruptly slowing down. The proton changes into a slow-moving, heavier W-boson which impacts the hull of the starship at non-relativistic speed and *perhaps* doing considerably less damage (see fig.4 below). Thus is the potential benefit of a Relativistic Nuclear Diffraction Shield.

Of course momentum is still conserved, so interstellar protons would still amount to 'air resistance' that saps the starship of its velocity.