Slack – How to Produce Unicode Symbols in Chat

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.

I haven't tried exactly what you're asking but I was able to use the API and write a post in a channel

https://api.slack.com/methods/files.upload

This could get you closer to what you want it to do