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Prefer to have discussions in the public channel, rather than in private: this makes us more open, and thus, more inclusive, with all the advantages that that brings;
Because of item 1, it’s okay to tag specific people on your public messages. A private message is no less annoying, and it can never be as directfully useful to the community at large as a public one.
If a discussion is particularly important, document it in a more permanent and accessible medium such as in a forum thread, a wiki page, or an issue (maybe even an OEP or ADR!) in the appropriate repo on Github. Slack is great for fast, efficient communication, but really bad at grouping and indexing it for future reference.
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