They want to take the things that are non-interactive about his kit (saving allies for free) and put power into the rest of his kit. He even said on Twitter that this isn't the last pass for Tahm.
He's slowed while eating allies (he can still protect them, and Phase Rush + Switfies is still a thing he can do to mitigate that), but he gained:
- A longer and more potent slow on his Q, with increased range
- More healing and shielding from his E, and it no longer needs mana
- Better damage on squishy enemies when Devouring them
- A better ult in the late game
This is just the first step. I'm all for taking the one thing that makes him very strong down a notch so that the rest of his kit can be better. Squad5 said that the intention is for these changes to be an overall buff for Tahm. Don't make harsh judgments until the full list of changes is out.