Can you delete a Slack message sent by Plain?
No. Plain has no delete button or API action that removes a message it has sent to Slack.
Slack only lets the author of a message, or a Slack workspace administrator, delete it. Messages sent by the Plain bot are authored by the bot, not by an individual user, so regular channel members cannot delete them in Slack either.
Ask a Slack admin to delete it
A Slack workspace administrator can delete the bot message directly in Slack. Plain picks this up automatically. It marks the message as deleted and shows "This message was deleted." in the thread timeline. The message content is removed from Plain, and only a minimal audit trail is kept.