I went to a theater to watch Mission: Impossible last night. It has a really long runtime (2h 48m) and, toward the end of the film, I started seeing multiple people pull out their phones for a time check; I had to stop myself from doing the same.
I think that has to do with excessively long movies and nothing to do with reduced attention spans. Very, very few action movies need to be more than 100min.
Anecdotal experience but we (n=2) didn't have the urge to pull out our phones on Oppenheimer (180 minutes).