In my company we use Skype for group chat (inb4 thanks for condolences). It has the edit/delete feature which I myself use mostly for fixing typos. But sometimes I'm a minute late to a discussion and see some deleted messages that are crucial to understanding what it was about. It's mildly annoying, and I was trying to reverse Skype to get at the deleted messages. It turns out Skype does store edits in its SQLite database, but sadly blanks out the original too if edit was a deletion. I'm too lazy to be bothered to set up a monitoring service that would make a separate, append-only log.