And for those not familiar with early Twitter, the hashtag was not a feature of the platform, but a shorthand way to indicate a topic. This of course was recognized and formalized by Twitter soon after, but I find it fascinating that it was a feature essentially developed by the users and more simply recognized by the platform later.
The part I quoted was about Twitter, not Slack. Slack's relation to IRC was already mentioned. Essentially, my comment was "Not just Slack, Twitter too was inspired by IRC."