If you destroy Illaoi's tentacles while they are about to swing, they will still deal damage, but you will not be able to see the hit area. Other abilities that have a cast time similar to the swing time do not go off if the champion dies before the cast time finishes, and I believe that the tentacles did not do this before the Illaoi tentacle change.
1. Have an Illaoi tentacle start swinging at you.
2. Destroy it before it slams into the ground.
3. Get hit by an invisible tentacle.