Check out Ripcord - https://cancel.fm/ripcord/. It's a third-party Slack & Discord client written in Qt.
Now, don't let the screenshots color your impressions as to what kinds of UI designs are possible under Qt: Ripcord's UI is bare-bones on purpose; it's a selling feature. I bring up this application because it lets you do a more apples-to-apples comparison: IM clients these days are a poster child of an Electron app that uses orders of magnitude more resources than they should for the functionality provided. Ripcord lets you compare the experience and performance with a native app, implementing the same features but without overhead and gratuitous UI shine.
(And personally, I do use it for Slack/Discord instead of official clients.)
Now, don't let the screenshots color your impressions as to what kinds of UI designs are possible under Qt: Ripcord's UI is bare-bones on purpose; it's a selling feature. I bring up this application because it lets you do a more apples-to-apples comparison: IM clients these days are a poster child of an Electron app that uses orders of magnitude more resources than they should for the functionality provided. Ripcord lets you compare the experience and performance with a native app, implementing the same features but without overhead and gratuitous UI shine.
(And personally, I do use it for Slack/Discord instead of official clients.)