Why was NWS shifted from green hydraulic to yellow hydraulic system in the A320? In older Airbus A320s, the NWS (nose wheel steering) was on the green hydraulic system and now it's on yellow. What's the reason for the change?
This enhanced package (green to yellow change) since serial number 1939[1] had the following objectives in mind:[2]

Objectives:

Avoid simultaneous loss of Normal braking and nose wheel steering functions on a single hydraulic failure. [normal braking is green]
Avoid loss of nose wheel steering function due to ADIRU or LGCIU failure. [ADR 1+3 failure results in NWS failure; ADIRU 1 and 3 control the Green shut off valve once the L/G is retracted and speed is above ~260 knots]
Make the nose wheel steering function available after a landing gear free fall extension. [L/G extension is green]


(My explanation is in square brackets.)
As a bonus, it would have permitted the eTaxi option (announced in 2013; scrapped in 2019). This way, the APU would have supplied power to the powered-wheels, as well as hydraulics for steering and braking (alternate), all without a tug, and without having to power the Green system via the PTU.[1]

1: Airbus FAST magazine #51
2: A320-family flight deck and systems briefing for pilots