Slack – How to export Slack conversation thread without admin account

To actually "slack-archive" the channels, ie the equivalent of running "/archive" within each channel, you'll need some API magic.

First, set up an API token for your user in the relevant team on https://api.slack.com/custom-integrations/legacy-tokens, then go to https://api.slack.com/methods/channels.list/test to get a json-listing of your channels together with the channel ids.

Then use these channel ids to run https://api.slack.com/methods/channels.archive/test for the channels you want to archive.

Here is a quick and dirty bash script (which uses jq) I recently ran on my Mac in order to archive the hundreds of channels that contained "sk-" in the channel name:

TOKEN="replace-this-with-your-user-api-token-in-the-relevant-team"
curl "https://slack.com/api/channels.list?token=$TOKEN&exclude_archived=true&pretty=1" > /tmp/channels.list
IDS=$(cat /tmp/channels.list | jq '.channels[] | select(.name | contains("sk-")) | .id' | sed -e 's/"//g')
for ID in $IDS; do
    URL="https://slack.com/api/channels.archive?token=$TOKEN&channel=$ID&pretty=1"
    echo $URL
    curl "$URL"
done

It got the job done, archiving about 2-3 channels per second.

The link that Slack provided for you and is now used in Trello as an "attached conversation" is a link into the regular Slack Archives.

Slack generally archives all of your conversations and provides deep links to them. The link you received when you attached the conversation to a Trello card should point to the exact moment when the conversation happened (albeit in the context of a larger, full archive page).

At the moment, there is no way how you could stop this particular behaviour - it is intended this way. "Attach conversation" does nothing more than insert one link to the current place in the Slack channel conversation archive. It doesn't actually include or copy or otherwise save the conversation.

You can however change the message logging in Slack itself: see https://get.slack.help/hc/en-us/articles/203457187-Custom-message-and-file-retention for this.