Our company has a similar number of channels, if not more, and you don't scan all of the channels for input. People typically subscribe to their team and related team channels as well as any topics of interest. Slack makes it easy to bring someone in just by mentioning them, so people get added to the places they didn't know about as needed.
Once you start seeing that you're subscribed to too many channels, you just leave them as they lose relevance to you. There's even a middle ground where you can mute notifications from a channel, but can still watch what's going on.
The main problem with slack is the poor search capability, and that's very relevant to your comment.
I get that you can selectively subscribe to only certain channels. The problem is that now you have 800+ channel names to read in order to determine the ones to which you might want to subscribe. Sure, someone else could invite you to some new topic-oriented channel if one remembers to actually do so. More often than not, in my observation, that doesn't really happen in a timely manner (if at all).
You join/are joined to a channel because its relevant to what or who you are working _now_. If you want help, then you go to #department or #thing those are the default public channels.
For example I was in a "SRE/devop/sysadmin" role, and I would be embedded in a production/feature team. So I would be part of sre-private and sre. I would also be part of #widget #widget-private and any sub feature teams.
This means that noise is neatly compartmentalised. I don't have to sift because its already in a labelled bucket.
