[notatoad]

I don't know about matrix, but you can import your slack archive into mattermost. So whatever the export is clearly well-structured enough to be useful.

https://docs.mattermost.com/administration/migrating.html?hi...

[RyJones]

JSON[0]. Here is the export I did for my employer, Hyperledger.

[0]: https://github.com/hyperledger-archives/slack-archive