I don't get why it was removed.
If I understand it correctly, Taliyah's W originally had a lockout animation because you needed to input the command twice - once to activate the ability, and another time to choose a direction. The lockout animation mainly existed for functional reasons.
After they changed the input to vector casting, the lockout animation technically wasn't functionally necessary because the spell now only required one input instead of two to make it work. So they removed the lockout. Apparently this also made the spell "feel better" at the cost of some game health, since this opened up the possibility for W->E combos when previously only E->W was possible.
I personally don't understand why the spell couldn't keep its lockout animation after a completed input. I feel like it's much healthier for the game and her kit. Other abilities have long lockout animations without "feeling bad", such as Lux R, Ezreal R, Orianna R, Braum R, etc. The return of W's lockout animation would encourage Taliyah's zoning potential more than her burst potential by removing the guaranteed E after a successful W hit, which would open up her kit to be more balanceable.
TL;DR I think it is completely feasible for Taliyah's current vector cast W to have a lockout animation post-cast and remain viable, which would make her much easier to balance overall.