For the last month...
Sunday to Thursday SLC-IAH-SLC travel this week on CR9 cleared at the window.
Thursday afternoon DTW-MSP travel two weeks ago on a 738 cleared at the window.
Monday afternoon SLC-MSP travel three weeks ago on a 738 cleared at the window.
Thursday afternoon JFK-SLC travel four weeks ago on a 738 cleared at the window.
I don't doubt that some people and some routes only clear at the gate but the vast majority of my upgrades this year have been at the window.
You're apparently the target. I wonder if DL is using/testing other unpublished variables re who/when/why upgrades are at the window.