I'm not convinced attention spans themselves have changed; I think it has more to do with the fact that people's brains aren't defaulting to daydreaming anymore when bored.
I think it's still an issue with people being addicted to phones because I'm sure the lack of daydreaming -- which studies have proven as beneficial and arguibly even a form of microsleep -- is likely causing a lot of the fatigue and inattention people feel; as in their brains are literally being deprived of a natural thing and so are exhausted because of it.
(as an aside: whew what a run-on sentence i have no intention of fixing)
My personal supposition with 0 proof is that the "gen-z stare" is the end-stage of this phenomenon where after being deprived of daydreaming for so long they are essentially "sleeping while standing up" as it were.