What about a 2021 Crosstrek wheels?You're right, that's a really hard specification to find. The closest I could find is a reference on 2015 and prior rims on an Subaru Outback forum. There a 2014 Crosstrek owner claims his rims weight 23lbs. Here is the link:
Crosstrek XV Wheels Fit on Outback? - Page 3 - Subaru Outback - Subaru Outback Forums
I'm not sure how accurate this information is. However if it is the 2016 rims likely don't weigh any less, as the "redesigned" rims likely involved very few changes.