If you don't like the office norm of Slack and wish it was email then too bad, same if it was the other way around too. It's now your problem to bridge Slack->email or email->Slack.
This is terrible woefully out-of-touch advice if the audience is anything other than executives who have enough clout to make the world move around them. If you're one of those people that is important enough that you can make everyone who wants to talk to you use your preferred communication medium then this might be actionable advice for you.
I swear we as a society didn't understand, didn't value the work administrative assistants and secretaries did (organizing all your communication being one of them) and we've been trying and failing to replace them ever since.
or risk forgetting to help you.
...or needing to scroll back through hours/days of messsges (ones that actually could be immediately answered or discussed, hopefully),
to find all the times they've agreed to help someone,
instead of you putting your (intricate, and/or technical request, from the context) into an email, ticket, or whatever you've all agreed upon.
I think that is the inflection point.
If you don't have a system in place, as an org, many people just don't think about it - find the closest, easiest text box, Send-Tweet. Or holler across the aisle of cubes at someone who is trying to concentrate...
Emphasis mine, this is literally the point I'm making but from the perspective of an office that agreed to use Slack. Which is most offices that have Slack. If email or a bug tracker is the norm then you use that, and you would be equally rude if in that situation you tried to make them Slack you instead because "you don't check email."
I know that's probably what their marketing department would like to hear, but that sounds horrific unless you're doing something quite mundane.
Which...if you're reaching out with a request time-dense enough that someone asks it be forwarded to email...it sounds like you're not doing mundane stuff.
etc
Oh, well the office also uses email. There are in fact places that use slack for time sensitive things and email for other things.