Thorn Trap applies its speed debuff to champions regardless of immunity to its damage.

Reproduction Steps:
1. Deploy a thorn trap.
2. Deploy a champion with immunity: physical. A Peaks Ifrit is a good option.
3. Activate the trap.
4. The champion will not take damage but will be debuffed. The speed loss applied this way also doesn't have a condition of its own displayed anywhere.

@Sokolov