Having been part of the steemit slack for a few years (and now the above mentioned slacks). Part of the myth were true, it was near impossible to contact the steemit team outside of said slack and by extension to receive a delegation.
This is no longer the case, slack is mostly people arguing about random stuff (threads of 500+ messages on whether climate change is real or not for instance) or sharing interesting posts.

Mattermost is where all the important dev conversations happen, but as gp said, if you're not a dev yourself, it's like watching people who don't speak your language. It's very uninteresting, for instance the last discussion was on how we should organize the javascript and python repositories and pipelines to generate (or not) some specific files in order to increase security of libraries.

Usually my use of those channels is like this:

I read everything every day on mattermost to be up to date with all the dev talks.

Slack is where lighthearted discussions happen, so I read it when I'm bored every 2-3 days.

Discord is the same but with 50x the people, I don't hang out there often because I can't keep up with the amount of messages (I'm kind of a completionist, I like to read everything), but whenever I get pinged I'll read the message and reply.