Since when did assassins become the dueling class

Since sometime in Season 3 when fighters and bruisers started a slow decline in public opinion. I guess people will never fully get over what Fratmogs and Force of Nature did to the meta way back when.

Since bruisers became ridiculously kiteable garbage? "Durr the well rounded class that underperforms in teamfights shouldn't be able to 1v1 the squishy teamfight dps specialist."

same time when "utility mage" became wave clear, 100-80 team, early,mid,late power, zoning class

As a funny thing, Garen would probably win a duel against ANY assassin with the exception of Zed, because Zeds core build is pretty much anti-tank items only: . So, he builds anti-tank, blows up squishies, and fucks up tanks.

While not "The" dueling class, their 1v1 ability, where they area meant to shine the best, makes the inherently powerful at duels. Also see Marksmen and Bruisers for powerful dueling champions.

Since when AD assassins existed and broke AD itemization in half

well, meleecarrys are still on top of the food-chain 1v1, and most bruisers can still stand their ground vs assasins(with current itemisation, Zed is more a melee-carry than a Assasin, unless the AS nerf does it'S job, but i think it was to small for that)