The key change is correct and cannot change back as it was, because of a weird bug in MSFS Simconnect we just found out, after users reported GSX froze when editing a custom pushback route.
When you subscribe for a certain key event, there's no way to differentiate between a Shifted and a non-Shifted version so, for example, if you subscribe to Shift-Up and just Up, if you press "just" Shift-Up, your code will be called TWICE with two Simconnect calls happening at the same time, so not only you don't know which event happened (both happened, from MSFS point of view), but the simultaneous call would cause threading issues, because pointers would had to be removed/deleted so quickly that their own Simconnect calls would mix up.
So, the only possible fix was to change the Shift behavior to be a Toggle, solving all reported issues of GSX locking-