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slackdump
Save or export your private and public Slack messages, threads, files, and users locally without admin privileges.
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matterbridge
bridge between mattermost, IRC, gitter, xmpp, slack, discord, telegram, rocketchat, twitch, ssh-chat, zulip, whatsapp, keybase, matrix, microsoft teams, nextcloud, mumble, vk and more with REST API (mattermost not required!)
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InfluxDB
Power Real-Time Data Analytics at Scale. Get real-time insights from all types of time series data with InfluxDB. Ingest, query, and analyze billions of data points in real-time with unbounded cardinality.
I've migrated from Slack to Mattermost recently. I used this tool called slackdump following the instructions here. I've managed to migrate several workspaces with accounts (including emails) and full conversation/attachments preserved.
As for the use case op hopes for -- the matterbridge project might be relevant. Ideally, you could migrate the history to a Mattermost server and set up the bridge. The Mattermost server will log all future communications automatically. The benefit of this over a pure archive (HTML or PDF type) is that the archive itself is a full-featured workspace ready to be used standalone at any time, should any issues arise from Slack.