The most recent I found was Mission Impossible: Fallout, a film I hadn't seen until yesterday and am only about halfway through.
Interesting because I felt it added zero value but, in full disclosure, I didn't care for this film in general. Despite a few decent moments, it felt painfully drawn...
After taking the good time and trouble to come up with what I consider an elegant and acceptable masking solution for my 16:9 screen I am now discovering many films that [for reasons I don't understand] use multiple aspect ratios. I've seen it in commercial theaters as well as in my home...