Anecdotal evidence etc. I have been having a feeling that my attention span is generally lower than before. I don't have a smartphone, no TV since more than 10 years ago, but there's the internet. Unfortunately working as a programmer requires constant connection (at least to be available to colleagues on chat, even non-remote). We've been experimenting with cutting off internet entirely for some days (e.g. yesterday), and I feel much better already. It's really magical how the nagging compulsion to check notifications disappears. No need to do any complex software setup, you go to the router and turn it off. So I think that quite clearly even with some mental fortitude and knowledge of the issue, you cannot really be immune to the attention-lowering effect.