Mordekaiser has gone 936 days without a direct change, behind only Zilean by 77 days.
Seeing the post about Mordekaiser mains "self-victimizing" and "whining" kinda pissed me off, so I went and checked how long it's been since morde was touched and how many champions had gone as long or longer without direct changes. Turns out it was 1.
6.1 for Mordekaiser and 5.21 for Zilean.
This is excluding a small effect added to Zilean's passive in 5.23, bug-fixes, and broad brush patches like 8.9 (mana changes), 8.3 (base HP cleanup), 7.22 (Runes Reforged compensation), etc.
His ult AI was slightly improved in 6.16 which would put a few more champions ahead of him, but I don't consider that a real change considering no actual stats were altered and his ult is still nearly impossible to control.
To add to that, almost all of his "crutches" for lack of a better word, have been severely gimped or removed. A lot of the reason he was good in season 6 was Rylais, which was completely gutted. A lot of the reason he was good in season 7 was SRS, DFT, Luden's, and Liandry's. SRS was severely nerfed in the form of Phase Rush, DFT was removed, Luden's was changed into a mana item, and while the reddit circlejerk would have you believe Liandry's is a god tier item on mordekaiser, in reality it's just not as good as the previous version due to the lack of magic pen, meaning most high-elo morde mains don't even build it until 3rd or 4th item. In pre-season 8 he was able to use Aery pretty well, but it was nerfed and is now being nerfed again, and his patch 7.22 base stat compensation from runes reforged of 5 armor, 5 base HP, and 5 HP growth was significantly lower than most other champions, especially considering how much power he lost by SRS being gutted and DFT being removed. In season 8 he was able to use Banner as a band-aid in top and mid since he couldn't stomp lane-phase as hard without SRS, and Shurelya's mid lane to exert more pressure due to his severe lack of mobility, but Banner was removed and Shurelya's was made into a mana item.
It doesn't seem to be getting any better either. With each new release/rework more and more mobility, cc, and crazy abilities are added, making the 325 movespeed mobility-less and CC-less champion that is Mordekaiser worse and worse to play.
Edit - Clarified some things.