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It's going to be a bit of a pain but what you can do is:

1. Disable all the users except one

2. Buy a license for the one remaining user

3. Export all your messages

4. Downgrade back to the free plan

5. Enable all the users again

Almost this exact sequence worked for me.

Wow, that's the exact kind of hack I expected from HN :) thanks!

Up until recently at least, you could still download all messages on a free trial of a paid plan.

The default export will download the text of all the messages, but it won't download any attached images (it will just have links to them). This tool doesn't export the whole history but it does export the attachments.

I think it might! Despite the Slack interface not showing messages older than 30 days, the official Slack export (even on the free tier) exports all messages. Slackdump might have similar access (didn't try).

It unfortunately does not. I just tried. It only has the last 90 days. You'll need to upgrade, export, then downgrade to get the full history plus the attachments.