his entire playstyle and uniqueness before the rework was that he was the only immobile, squishy melee champion with no sustain (by squishy i mean no skills that allow him to mitigate damage dealt to him) and he worked best lategame as a powerful duelist and splitpusher. His itemization was mostly offensive with only the defensive items you absolutely needed as the game went on
Reworked Darius has sustain based off of his health pool, and tons of free AD in his passive, making defensive itemization far more efficient (and desirable). his gutted attack speed, removal of the attack speed slow damaged his potential as a duelist by a lot, while the Q cast time preventing him from using basic attacks during the windup and lowered W ratio made his tower pushing power far weaker (by also making triforce go from his core item to being really bad on him)
they took a champion with a kit that has horrible teamfighting and forced it to work by implementing a gimmicky ball of stats passive