Jhin is meant to be immune to AS slows

Because he has no scaling AS to build. On top of this he is meant to be a damage champ, and often you actually lose DPS by converting pure AS into damage. 2.5% damage for what is normally a 10% increase. But he gets to scale on his abilities with it. So actually, he is eating almost a flat up debuff to get extra damage off crit chance which even then causes him to crit for lower than normal damage. So you want to complain about someone getting a buff to make up for the fact that they scale lower than most ADCs on the things that have infinite use while having higher scaling on things that have limited use (abilities).

The solution to Jhin is not an AS slow. It is armor and hp. He does abysmal against tanks until extreme late game. Building an AS (that actually works against Jhin) is like building a Spirit Visage to counter Vayne. Sure you do get some benefit out of it, but it has wasted gold and and there are better items for the job.

Edit: I just realized that if you think about what would do to his damage, this would be a very bad thing to add to his kit. It would take away his AD and reduce his damage therefore super gimping his damage on both AAs and abilities.