its 10% less dmg from projectiles, which implies ranged dmg, though not necessarily all ranged dmg is projectile based (blastbones is technically a ranged attack but does not use a "projectile" as one example, another case would be degeneration from mages guild, this has a projectile but the projectile itself does no dmg, as it applies a dot)

the problem is that mitigation has diminishing returns, so you could say have major protection (10% dmg reduction against all dmg) and this, but you wont be taking 20% less dmg from projectiles due to the diminishing returns, it would only be something about 16% or so less dmg against projectiles

changing it to something else might might make it too OP though

if it was changed to 10% against all ranged dmg, that would probably make people too tanky, it might potentially be less OP to be protection against melee dmg, but that might also have the unintended effect of making ball groups more tanky anyway due to being less susceptible to bombers, especially if it stacks with other mitigations. these might be more feasible if the bonus was reduced to say 3-5%.

changing it to major protection while it wouldnt stack, would also be pretty OP to give everyone that much flat mitigation to everything

no point changing it to minor protection either, because you already can get that from scribing pretty easily

right now it does kind of provide a strong bonus against some of the stronger ranged attacks, such as merciless or overload both of which use projectiles and 10% dmg reduction on those plus other mitigations make it easier to absorb the hits

Generate an aura that instantly kills anyone wearing Rushing Agony.

The set is 100% used in PvP, mainly by some ballgroups. Was a bug where it would reduce the damage from azureblight (back when it was used in PvP before the unnecessary nerf it got) since the game somehow considered the azureblight damage a projectile.