Thank you for your continued feedback. We are currently working on this, and recently announced this feature for developers in Beta. For more information, please visit https://developer.microsoft.com/microsoft-teams/blogs/importing-3rd-party-platform-messages-to-microsoft-teams-is-now-available-in-beta/.
An error occurred while saving the commentKyle Darling commented
Please do this! Seems like it should have been done a long time ago since history is so important to Slack/Teams/Hipchat, etc.