Every new act, episode, or agent that comes out is followed by a literal flood of smurfs, who actively admit to smurfing (can't report someone for it so what do they care).

For example, Chamber most recently came out. First unrated game as a 4 stack of account lvl 60-100s, we faced an entire team of under account lvl 15 who stomped the hell out of us. Highest level played chamber, claimed "I have exams and no job, I don't have 3 days to unlock him on my main, it's unrated stop complaining". Come to find out he is immortal on his main (our 4stack were 1 bronze, 2 silver, and a plat).

But because they are new accounts they just pubstomp both unrated and comp so they can play a new character every single time they come out. Yeah I said comp too, because playing 10 games is easy for an instant lock immortal Jett or Reyna to go in, stomp, and progress.

Now the obvious answer for Riot would be to add something similar to Prime Gaming on CS:GO where you need a phone number attached to your account and each phone number can only be used once ever.

OR

For the love of everything take new characters out of the "free agent" unlock pool for a certain amount of time.

The game is literally unbearable to play whenever a new character comes out solely because of this. Let alone the sheer amount of Smurfs naturally around. They fixed a tiny part of this with the new 5 stack changes, fix more.