It's not equivalent. The getter-only auto-property will not have any setter at all. Previously, it had to have at least a private setter, which means that you couldn't strictly guarantee that it wasn't being updated. I used to write read-only properties the long way just to guarantee that they were actually read-only.
I often write stuff like this:
public RxProperty<bool> IsFoo{get;private set;}
I could do without the private set.