Nerfing Garen's mobility is the wrong way to nerf him.
It's already so low with the previous changes that Black Cleaver was forced onto you, and with the new mobility nerfs you're now forcing Deadman's Plate onto him just so he can even have the chance of getting near a player. Those nerfs are going to hurt him through the entire game and his early game is already weak enough; why hit him more there with nerfs obviously targeted toward trying to make his late-game weaker?
Just change how his spins work; make it scale from 6-9 instead of 5-10 (And get rid of the 1% max HP true damage on autos). That'll lower his damage late-game so you guys don't have to continually cripple him throughout the entire game. He's already such an immobile champion that any further changes to his mobility is just stupid and unjustified.
Unless there's some context on this change? If Deadman's is so freaking strong, then nerf Deadman's; don't nerf Garen BECAUSE of Deadman's.