Here [0] is a link to the slack export documentation. Slack not providing an export to the provider of your choice isn't necessarily slacks issue, it's Matrix's issue.
> I tried the data export from Asana once and it was useless.
I don't think you can infer that all SAAS platforms lock your data because you had a failed attempt with another such tool once.
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.
I assumed all these SaaSS products are built like carnivorous plants - The spikes only point in.
I tried the data export from Asana once and it was useless.