You're missing the key modifier: legitimate. It's not an irrational fear of missing out; it is a very real concern about a culture where being less responsive on Slack reduces your influence on key business decisions. "Close the app" is only good advice if you both want to and can afford to opt-out of any involvement in those decisions.
Agreed. There are times I only join conference calls to ensure that no work gets assigned to me without me hearing about it first, or that I have the ability to chime in as work is being assigned. I implemented this policy when it became clear that work was being assigned to me (imo incorrectly) because I wasn't there to say no.
If decisions are being made over Slack, I have to be involved or suddenly I may find myself responsible for something I have no control over. Simply because I was not there to let people know who actually is responsible.
Call me rigid, but isn't the ability to overcome this kind of silly stuff exactly what separates well-adjusted adults from everyone else?
When you catch yourself doing things out of FOMO, take your dispositions and fix it. In this case: close the app, or configure the alerts.
I'm sometimes alarmed by how people seem to have zero self-knowledge and appear to run on auto-pilot, guided by base impulses.