This is a fun rune but ultimately not generally good enough if you want to actually win games. I don't know if it's optimal on literally any champion right now. Even Xin doesn't take it anymore.
This has the potential to be abused by ranged champs since they can proc. it from safety without even getting in range of CC. However, for melees, it's not really possible to use it as safely.
It already has slow resistance and my buff proposal is to give it 25% tenacity for melee in addition to the 75% slow resistance for both melee & ranged. I think this would finally make it a realistic option instead of just kind of a troll experiment. Before you get scared about Yi taking it, know that you still give up a ton of damage not going Lethal Tempo, PTA, Conqueror etc. on the champs which would seem scariest with this. A Phase Rush Yi would also have strong anti-synergies with his R because of MS caps in the game drastically reducing the effectiveness of additional boosts + the slow resistance on it would be wasted on him. Yi needs to be strong 1v1 and to clean up fights. 25% tenacity won't suddenly make it worth on him or champions like him.
I was also thinking that it might be nicer if it gave flat MS so as to not overly advantage champions with high base MS. What do you guys think? Have other ideas?