The assassin buffs were meant for patch 12.11 after the class was driven into the dirt with the durability update. Still, better late than never, and the class is hard to balance as is.
The durability update Riot Games released in League of Legends in patch 12.10, made snowballing the game by killing enemies early extremely hard. This is due to the defensive stats, especially armor, being buffed considerably, giving the squishy champions a lot of survivability early game, with no extra gold needed.
Of course, each of these things is a direct nerf to the assassin class, and when put together made AD assassins fall out of the meta completely.
To remedy this, Riot games are finally implementing the buffs meant for patch 12.11, after some further testing was done. this time, the buffs are going through the Mythic items:
"Assassins are currently doing a balanced amount of damage with lethality items, but the class overall is just a tad weaker than we’d like. We’re taking a non-traditional approach to buffing assassins by adding bonus move speed to their Mythic iItems which should also help discourage bruiser or tank itemizations."
This means that, for Duskblade, Prowler's Claw, and Eclipse, 5 Ability haste is joined by 5 move speed as well. This should help the assassins move around the map more, and achieve their snowballs by ganking other lanes, and not by killing their own laners as easily as they did previously.
In essence, the assassin class should finally go back to what it used to be in the golden days of League of Legends.