quick cast is part of the controller input system.
in other words quick cast uses the same input system as the controller.
so all’s they needed to do was tap into it and make a UI for it then figure out how to activate the new skills.
So far we have seen lots of UI and Input system stuff.
Those features are built-in to game engines like Unity for instance.
but normally these guys use in-house tools and stuff for work flow and not unity lol
I don’t believe it was as much community feedback as a normal player would think - more like the marketing team advising the community managers to mention it to influencers who then make videos explaining how they want changes X_ to go.
then they throw up polls like this:
too much on the line to just do whatever the plebs ask for 
if they did, I’d be nothing but a wall of spam from people asking for everything all the time.
Not to mention people asking for gasoline to add to a fire.