If you click on the message section on the YouTube, you will see a notification that informs you that the section will no longer be available after September 18.
It says “Two years ago, we launched a feature to enable you to share videos via direct messages on YouTube. Since then, we’ve also focused on public conversations with updates to comments, posts, and stories. We’re constantly reevaluating our priorities and have decided to discontinue YouTube’s native direct messaging feature while we focus on improving public conversations”.
Google has also revealed that some features such as sending videos as messages will be discontinued even before September 18.
We have no idea why Google has decided to take this step, but our guess is that the message section isn’t gaining much popularity as people barely use the feature except for sharing videos.
However, it stated that users would still be able to share videos by clicking on the share button below the video and then clicking on “social network icon”.
If you would like to download a copy of your messages before the feature expires, Google has provided instructions to do so on its support page.