Hi all
great thread!
If you use a custom solution that provides embeddable JS, you could potentially pass the selected variant and then use that to dynamically show/hide parts of your upsell page.
So yes, technically possible via custom JS/CSS, but it would require some custom implementation on your end.
For anyone still looking into a custom solution for hiding parts of the upsell page based on the checkout purchase,
I found a great solution for implementing conditional upsells here.