I don't know where is the problem in the code, but
public interface OnHitEffectPlugin {
void onHit(DamagingProjectileAPI projectile, CombatEntityAPI target, Vector2f point, boolean shieldHit, CombatEngineAPI engine);
}
don't detect shieldHit properly anymore.
[EDIT] False alarm, sorry about that, but it wasn't really my fault, I swear! Apparently Netbean don't really recompile your code if you don't modify your file, but use the previously compiled version. I have copy-pasted the exact same code from an old file into a new one, and it work in the new file, not in the old..... I should probably clear some cache or something to avoid strange issues like that.