A lot of people talk about Ekko as an assassin. He's honestly more of a "light fighter" than an assassin. His kit is more similar to Fiora's than Zed's or Khazix's.
Like Fiora, he's extremely mobile, has clutch survival tools, and lacks burst, but can hit-and-run well, and deal a lot of sustained damage.
The thing is, Fiora has gone through a similar cycle. She built almost straight tank (with 1-2 bruiser items) and crushed people. She was incredibly slippery and almost impossible to peel. After months of nerfs, the tank build seems pretty much dead, but it doesn't seem like Fiora has any build to fall back on. People didn't switch from the bruiser build to a more damage-oriented build, they just stopped playing her altogether.
Even Yi (another light fighter) is in a similar boat. Once the tank build was really killed, he disappeared back into obscurity. Although losing devourer is also clearly a factor here.
A lot of people point to Ekko's crowd control as the problem. But a lot of the other light fighters seem to be in the same boat, despite lacking Ekko's CC.
The light fighter class just isn't working. Players are either face-rolling with much heavier item sets than Riot intended, or not picking the champions at all. I don't know why that is, but it doesn't seem like the problem is unique to Ekko.
I think Riot needs to look at the class as a whole and figure out a way to make a light, hit-and-run kit actually work as intended.