It's not Slack's fault. It is a good platform for one-off messages. Need a useless bureaucratic form signed? Slack. Need your boss to okay the afternoon off? Slack. Need to ask your lead programmer which data structure you should use and why they're set up that way? Sounds like the answer should be put in a wiki page, not slack.

All workflows are small components of a larger workplace. Emails also suck for a lot of things. They probably wouldn't have worked in this case, memos are the logical upgrade from emails where you want to make sure everyone receives it and the topic is not up for further discussion.