If you've got the kind of social life and communication you need, great. And I agree that many people who are in an accidental too-much-FOMO situation probably didn't need all the extra tweets and whatnot flung their way and it's mostly junk. In that case, just turning most of it off is a reasonable idea.
But if you're in a position where the communities you need to be making more of an effort with use fast-moving, presenceless platforms for most of their socialization (Telegram, Twitter, etc.—maybe even, say, Slack?), good luck snagging those limited opportunities to learn which people to get to know better or get a word in on something important without constantly checking everything. (This might be a good argument to not do that for your next community!)
The idea of more closely regulating the feeling so that it's not overdriven and occupying your whole mindset is still relevant, but it takes on quite a different form if you can't actually afford to check out of the fast side; you have to do a trickier balancing act.
“Hey, I like what we're doing, we should set up a group for it to make it easier to keep up with events and stuff.”
“Telegram? That's what everyone else seems to use.” “Yeah, I've already got that on my phone.” “Me too.” “Yup! And I've got this great sticker pack I want to show you…” “Hey, can you show me how to install it?” “Sure, it's easy, just…”
“Awesome, I'll set it up.” (tap, tap, tap)
(Substitute Facebook, or Discord, or whatever else.)
Gradually people just kinda forget to post things anywhere else, because it's convenient and lets them post from their phone and they get all the encouraging responses they wanted there, and so it gradually becomes common knowledge that that's the place. The choice of platform isn't in their community identity, so the default is whatever people gravitate to that doesn't require doing something unusual.
If you don't integrate psychologically with the popular platforms, you're not really choosing to not be part of a specific community, because that's what “popular” means: you're choosing that the set of communities you can explore in the first place is whittled way down, and your membership in any you do find is now precarious. “Pick a community that doesn't do that” not only raises the difficulty at the beginning, but it also doesn't save you later.
It seems to me that message notifications, at least, do keep me from checking my phone.
If that's not the case, are you really sure that you need to be doing any of that? Especially "get a word in on something important" is awfully similar to https://www.xkcd.com/386/ .
Dedicating a time slot in the usual sense is very socially costly if they've gravitated to a presenceless, fast-moving platform, because psychological consensus and topic closure operates more on the perceived speed of communication of the group. If 90% of the other people respond within an hour, and you get there eight hours later, the conversation's moved on, so if everyone else is in the habit of checking their phone every fifteen minutes, there's pressure for you to be too. So, yes, you can “dedicate a time slot” at the end of every pomodoro, if that's what you meant.
This is more true in “channel”-based environments where “new in thread” is inconvenient or unavailable. If you're not careful, you can even be disruptive. Even on slow-moving, old-style Web forums, “necroposting” is considered rude; a chat-like medium with little to no threading support can just lower the threshold for it from weeks to hours if there's enough activity.
For the other part, I'll grant that “get a word in on something important” was badly worded, though I can't think of a better phrase this instant. I was imagining things like “we've changed the meetup location because someone raised a problem, is this okay with everyone or does someone need a ride” or “I'd like comments on which direction I should go with the next part of this piece” or “I'm going to see if some of us can do X together tomorrow, I want three more people, who's with me”. All of these can easily render your participation irrelevant if you show up too late, and “too late” is by default defined by what's usual and convenient, not by abstract considerations of what's good for people's habit formation.
If they're doing it only because they kind of fell into it as baseline socialization shifted, they need to step back and reconsider things because it's not healthy.