I've played the game since beta.
Top lane rules 2-3 seasons ago
Mid always took top 1-2 seasons ago
Jungler hitting 6 before anyone else- I honestly forget this one
Assassins one shotting? Talon, Fizz, LB - these go WAY fucking back. 3-5 seasons ago, especially when were talking about when LB and Talon had silence, yikes
Tank meta arguably still huge, so agreeable
ADC's were weaker 2-3 seasons ago
People could "truly" solo carry anywhere from 2 all the way back to 5 seasons ago
Here, since that clearly wasn't enough:
-Remember when the champions started getting mobility --> Tons of bitching (Mobility creep dates back 2011 I believe)
-Remember when Devourer was released? --> Tons of bitching (albeit rightfull so, 4 seasons ago)
-Remember when people constantly bitched X Y Z mastery --> Tons of bitching (there were so many of these ,just look through archives)
Older stuff is harder to remember, but if you want me to point out "cancerous" things that weren't recent that people bitched about, I'm sure I can dig it up. My point here, which I think you entirely missed again, is that there will always be a minority, and sometimes majority- that aren't going to be pleased because it doesn't fit their agenda or what they want. Often times these people hold tons of bias for their own champion, items, or success.
And look, I'm not saying this game is fucking perfect, it's far from it. But I'll be damned- I've had more fun in the past 2-3 years playing LoL than I can remember having in the prior years. Sure part of the fun back then was there wasn't a meta. It made the game exciting, experimental, and innovative. But with any good game there begins to be meta, strategies, and tactics that evolve the game into a simpler form. I think that's OK too in its own right. You begin thinking what your current situation is and you have to devise a strategy based on that. In a lot of ways it strikes a lot of resemblance to chess or a RTS.
P.S. Generally agreed is such a fox argument, proof? I'm 99% sure I can find old quit threads dating years back that consist of people thinking the game was declining then too. My point here is you're just looking for some observation bias that fits.