There are a couple hundred people at my location, which is a Slack workspace. Each project gets its own channel. Each department (regardless of project) gets its own channel. Depending on the size of the project, they might make their own subchannels (#project-qa, #project-business, etc). There's a buy/sell/trade channel, a running channel, a football pickup group channel, a video games channel... Likely several other casual channels I don't know about because I don't search for them.
I don't think it's noise, because really most people will only be in #general and #project. Having a solid notification workflow is more important to me than channel workflow. I set my Do Not Disturb to be from like 2am to 9am, since that works better with my flow, have mobile notifications on, and mute basically everything other than my #project.
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