When you delete a completed Savings Goal the contributions to that goal in previous months' Spending Plans also disappear.
When those contributions disappear from the Spending Plan, it makes it look as if I had much more money "Left this month" in each of those months than I actually had. That makes my Spending Plan look seriously out of whack and erases an important part of the planning that was done.
This appears irreparable, as you cannot recreate a goal from six months ago and go back and make contributions in each of the previous months.
Currently, there is no way to hide Savings Goals that we longer need to see. They just build up in the "Completed Goals" section of Savings Goals. And heavy users can end up with pages and pages of Completed Goals. But if you want to get rid of them, you have to delete them.
(Here's an example of a goal I no longer need to see. I created a goal to set aside money for semi-annual property taxes. After the goal has been completed, the funds withdrawn, and the taxes paid I don't need to see that goal again.)