They are making his W only castable on ally champions (so you can't actually solo-lane anymore) and it won't deal damage unless you are near your ally.
They are nerfing the base damage on his E, making it only useful for charging shield.
They made his primary damage source (Q) reliant on laning three autoattacks against a target as an immobile melee champion and they further reduced his movement speed.
These changes also gutted his waveclear and his ability to farm jungle camps.
If these changes go through, Mordekaiser won't be able to play in a solo-lane and he'll be ham-fistedly shoved into being a gimmicky bot-laner.
In addition, the changes to his W (experience and damage-wise) are designed to force him into a duo-lane, whether you wanted to be or not. The closest they've come to forcing a duo-lane so far are Kalista (Soul-bound) and Leona (her passive) and even they are capable of laning solo if they desire.