At work we started doing private project-specific channels for whatever initiative we're working on that's cross functional. As soon as that project is complete

At work we started doing private project-specific channels for whatever initiative we're working on that's cross functional. As soon as that project is complete, we archive the channel and move on the next thing. It's worked out really well surprisingly. Our public channels rarely get activity now because their subject matter is so vague but the private channels are incredibly active and productive.