No, there are plenty of kill mechanics in the game. The game doesnt have any "unavoidable" kill mechanics, but plenty of things end you right away. Falling off the island in limitless blue? Dead. Didnt look away from Angra Manyu's gaze and didnt clear the resulting Doom debuff in time? Dead. There are plenty of fights with soft or hard enrages too, so the people who play too poorly to finish the fight in time? You guessed it, dead. But none of that is unavoidable. Dont yeet yourself off the edge, do the gaze or doom mechanic right, do enough damage to win - each instakill mechanic has a proper way to be done that ends in you not dying.
So when OP mentioned Orbonne i was curious which "death" mech they were going to incorrectly complain about. As they described on, i was guessing either Crush Weapon or Crush Armor. Then they mentioned the "white beam that keeps hitting you" and that narrows the move down to Crush Weapon. The description is a poor one - the attack is not a beam, its a giant sword slashing up from underground. And it doesnt "keep hitting you", it attacks 3 times period. But most importantly, its totally avoidable. TG Cid calls out the attack verbally, his cast bar shows the attack name, and the person(1 per group) to be targetted gets a marker over their head. THIS is your sign to move(and for the rest of your group to get out of the way), not the orange AoE circle appearing. You start running(no sprint needed) before the aoe appears, and it never catches up to you. No damage taken, no "unavoidable death".
Edit: I still interpret the description as being Cid's Crush Weapon. If Vyrerus is right and they actually meant Agrius's ruination that makes even less sense. That attack is both avoidable(do the shield mech/dont stack on the person targetted), and doesnt even deal lethal damage unless you have a few vuln stacks.