Is there actually a tier list out there or are they just speaking about something non official?
I've seen different youtubers and so put out tier lists of their own, but the constant use of tier list by the casters had me wondering since I don't even remember hearing them really say this other splits
It's theoretical, mostly based off of preferences by different teams throughout the current patch. For example, despite nobody having put out an official tier list, there's a relatively clear list of Varus/Jhin, followed by Ashe or Sivir, and then maybe Ezreal, MF, or Cait. That's just what we've seen recently, and therefore what the current tier list appears to be.
This is kind of a good tier list which usually is pretty accurate as a generalization. Mind you this is more intended for solo queue performance and not necessarily pro play or with a premade team, so don't expect every champion to be exactly how they are ranked. I usually give everyone a +/- 1 tier bias.
But, most of the time it is usually obvious as to what is strong and what is not. Just look at pick/ban percentages, the closer to 100% pick/ban, the better that champion is (this is all relative to pro play mind you).
no tier list would ever be official, they just refer to how pick/ban priority has been shaping out over the games on this patch (e.g. Jihn and Varus being the most contestet carries although Varus rocks a 36%winrate because teams have "analysts" that can´t interpret statistics to save their own soul apparently cough - so they are considered S-tier).
