Migrating to Mattermost from Slack and transitioning all the data is easy. My team at work transitioned to Mattermost a few years ago. https://docs.mattermost.com/administration/migrating.html#mi...

Thanks. This seems to be the important part:

> Generate a Slack “Export” file from Slack > Administration > Workspace settings > Import/Export Data > Export > Start Export.

If the account is already closed, this is not possible. And this is a major problem.