This guy hit the nail on the head. Many many many, of the older champs that are not played anymore, are not played because of about 4 reasons.
1. Low mobility(not a huge problem, if they have cc and/or escape)
2. Low cc
3. They are hard to play as(malz q is kinda hard to hit right)(also they can be high risk high reward. But because they are hard, many people just get the risk part, and not reward(because the champ is hard and they cant play them properly
4. They dont have a escape.
The main reason champs such as malz are not played anymore is because, they are low mobility, and dont have an escape ability to compensate.
Right behind the main reason, is simply the skill needed to use them properly. High skill champions are usually played less.(or not as well at least)
A champ with low mobility and no escapes, AND a high skill cap, are usually not played.
As proof that its not that the champs are weak, but that they are just low mobility, and no escapes look at this.
Who were the most played champs mid season 3? Ahri and zed and assasins. What are assasins? High mobility champs.
Who were the least played champs marksman? Kogmaw, MF, Urgot(yeah i know he isnt a real adc). Low mobility champions.
So. Malz is still very very strong. Clease doesnt work on his ult(last i checked)(but qss does) His weakness is no hard cc on his normal abilities, no escape, and he is a bit slow.
What they all have in common is that they were highly popular / at least played in Season 1 and / or 2 + lack the ability to reposition themselves, thus are not able to escape clutch situations that easily.
Not having such a skill means that there is already a higher skill cap and other disadvantages because
→ you don't have that get-out-of-jail-free card
→ not being able to reposition yourself means once you commit to a fight there's more or less no way out, unless you have Flash or Thresh
→ high mobility makes it easier to juke, kite and kill high priority targets
.. the list goes on.
Personally I enjoy high mobility champs much more, so I'm not ranting or anything. It's just kinda sad cause to me it seems that low mobility champs have that clear disadvantage and the higher ranked you are up to competitive play you can see that even pros value higher mobility in most cases.