Ripcord will temporarily remember deleted messages for channels in which you're a moderator. You can view the contents by right-clicking on the message. It won't do this if you aren't a moderator in that channel.

This is to help avoid having to use a bot to log all message edits+deletes, usually done to mitigate bad actors saying something bad, getting someone else to report the message or take a screenshot of it, then delete it afterwards.