You want to know which patch note made Leblanc broken?

v3.09 LeBlanc's Mimic was less a decision point and more of a tool to one-shot someone with Sigil of Silence. These changes aim to make Mimic equally interesting with other skills. For one, Mimicked abilities will now deal their own base damage instead of amplifying the base spell. This means players will be able to use Mimic in more creative ways at level 6, rather than saving it for Sigil of Silence. Additionally, Sigil of Silence will now deal less damage up front but more damage on detonation in order to better promote LeBlanc's multi-skill interaction.

Mirror Image The Mirror Image will now attempt to run instead of standing still Sigil of Silence Mana cost reduced to 50/60/70/80/90 from 70/75/80/85/90 Damage changed to 55/80/105/130/155 (+0.4 Ability Power) for both initial impact damage and mark detonation damage Impact damage was 70/110/150/190/230 (+0.6 Ability Power) Mark damage was 20/40/60/80/100 (+0.3 Ability Power) Silence duration reduced to 1.5 seconds from 2 Distortion Cooldown now starts on cast rather than after returning to your Distortion Mana cost increased to 80/90/100/110/120 from 80 Fixed a bug where Distortion was visible in the fog of war Ethereal Chains Slow duration (and time until root) reduced to 1.5 seconds from 2 Root duration changed to 1.5 seconds from 1/1.3/1.6/1.9/2.2 Cooldown increased to 14/13/12/11/10 seconds from 10 Mimic Mana cost reduced to 0 from 100/50/0 All mimicked abilities now deal their own base damage instead of amplifying the base spell's damage Mimic: Sigil of Silence Now deals 100/200/300 (+0.65 Ability Power) damage once on impact and again if the mark is detonated Mimic: Distortion Cooldown now starts on cast rather than after returning to your Distortion Now deals 150/300/450 (+0.975 Ability Power) damage on impact Mimic: Ethereal Chains Now deals 100/200/300 (+0.65 Ability Power) damage once on impact and again if the target is rooted

This patch was a huge deal for Leblanc, it made so that people would max W instead of Q. Her old ult allowed her to deal 40% damage increase of the abilities she copied meaning she would always max Q then W. If riot revert her to her old Ult numbers it would fix leblanc and she would have clear lane weakness. She would have less safety and would deal more damage if she maxed Q. She would have less waveclears too.