Tahm Kench is being forced into a Top Lane role which is not where he shines. Don't get me wrong, I love Tahm Kench top. I play it all the time and its a great pick below diamond. But diamond and above Tahm Kench top is no longer underestimated and is generally avoided. That being said, he falls off late game if he doesn't get a good amount of kills, and his only merit late game is his utility. (His R and W to save people.)
Even if you don't get fed when playing top kench, you still feel like you're able to do something late game because of your tankiness and your "Armored Personell Carrier" gameplay, saving the carries from certain death. But if you put the 95% slow when devouring allies in lives, he will be virtually useless if he doesn't get fed. All of his abilities are single target spells meaning he can't neutralize more than one person at a time. With that in mind, he's one of the most lack-luster tanks in the game. All he can be is a bullet sponge, but his damage leaves a lot to be desired. Couple that with his lack of gap-closer, hes vulnerable to being kiting heavily in high ELO. Despite being a tank, a Tahm Kench who is the first to engage is a bad Tahm Kench.
His saving grace and what makes him not a troll pick is his ability to pick up a friendly champion and skedaddle. I've heard people say its overpowered, but I don't believe it to be any more overpowered than Thresh's Lantern, because Thresh doesn't need to put himself in harms way to save somebody. With a 95% slow when eating friendly champions, and the inability to ult with him if you're in combat, he stops being a savior, and becomes a second Zhonyas hourglass. With such a high drawback to eating friendly champions (What with the cooldown now being longer if you eat a friendly champion) then Tahm Kench's will stop being able to use versatile builds to help the ADC. A good tahm kench will build Righteous Glory to help the ADC catch enemies if they are ahead, or Shrelyas to help ADC's get away if they aren't. With a 95% slow when eating champions, its almost useless to build either of these items.
Tahm Kench is one of the worst Supports right now because of how much time you have to play him in order to be able to play him well. I've spent hundreds of games playing Tahm Supp and its the only place Tahm can truly be appreciated. If you slow him when he eats friendly champions, you're just turning him into another bullet sponge. But unlike any other tank/CC supports like Thresh, Alistar, etc. Tahm Kench has no AOE, and can be almost entirely ignored by the enemy team as long as the ADC's kite him.
Don't turn Tahm into a useless champ. His whole identity is being a savior. If you put a 95% slow on Tahm's W when eating allies, eating an ally won't save them, it will just kill both of you.
Begging you Riot. He's my favorite champ. His support playrate has gone down significantly ever since you removed the movement speed bonus on his W when moving towards enemies. He will reach unplayability, and become a borderline troll pick like Teemo if you do this to us.