Sorry, but most of that is simply not true. I worked for a large news company that moved from virtually nothing to slack. There is now something like 800 channels. Each channel has a purpose that is distinct from each other (for example system-outage-44, or graphics-team, or aws-help) Each team normally has a public and private channel, one to provide support, the other brisk frank discourse. Seniors are expected to…
Having to sift through 800 channels to find the relevant topic sounds like a nightmare.
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.