So because Assassin meta's happen they should NEVER be viable in any way shape or form. Meaning every assassin should have an abysmal winrate?
Okay, so, in my opinion the winrate of a champion should correspond with their Playrate and Learning Curve. Assassins have pretty high learning curves on average, their just as squishy as most other damage dealer's but don't get the safety of range(usually, Leblance and Ahri are exceptions.) which makes them harder to learn and makes player's put more time into them. Because of this assassins should have low Playrates, how you know were in an assassin meta is when ALL ASSASSSINS NOT JUST THE AD ONES have play rates above 10%. Out of 13 Assassins only 3 had pick rates above 10% those champions are as follows in their primary roles.
These three are heavy outlier's, as you can see.
Kha'zix: 18.47% pickrate 51% winrate
Pyke: 13.85% Pickrate 49.04% winrate
Leblance: 10.34% Pickrate 48.49% winrate
5 had pick rate's of over 5%, a couple might need some minor nerfs but this is definitely within the lines.
Evelynn: 50% winrate 5.7% pickrate
Talon: 51.41% winrate 5.27 pickrate
Zed: 6.95% pickrate 50.75% winrate
Ahri: 51.2% winrate 5.55% pickrate
Akali: 47.6% winrate 5.44% pickrate
The rest had pickrates bellow 5% These champs are used only by mains.
Katarina: 52.04% winrate 4.11% pickrate
Fizz: 51.33% winrate 3.24% pickrate
Ekko: 50.82% winrate 3.56% pickrate
Rengar: 49.77% winrate 1.94% pickrate
Shaco: 53.82% winrate 2.72% pickrate
There's more assassins than this but can you tell with ease which ones need some tuning? Obviously the top 3 up there need to be nerfed, but for all the other's it's kind of hard to tell, assassins are among the funnest to play champions in the game and their playrates have always reflected that. Right now only 3 out of 16 total assassins are seeing play rates above 10% and those 3 are heavy outlier's as well, this is not an assassin meta.