From December 15, the Messenger app on Mac and Windows devices will cease to exist, and the company has already removed it ...
Starting December 15, users will no longer be able to log into the apps and will be automatically redirected to the Facebook ...
If you're using Messenger on macOS, you'll want to switch to something else — and probably sooner, rather than later.
The desktop Messenger app is being retired, but chats will still be accessible through Facebook and Messenger on the web.