YouTube direct messaging will be discontinued on September 18th
Google has announced that it will removing the ability to direct message other users from YouTube on September 18th. After that date, the functionality will no longer be available.
The YouTube Help page that announced this upcoming discontinuation stated that the decision to remove the feature was in order to "focus on improving public conversations," with a particular focus on commenting, posting, and story functionality.
Though direct messaging is going away, you'll still be able to share videos via social media and direct linking using the messaging service of your choice. Similar direct messaging functionality using large social networking sites can be found via Twitter and Facebook Messenger, though more secure messaging services such as Signal and Wire may be better options if you can convince your YouTube direct message friends to move with you to the service of your choice.
For those that want to save a copy of their message history from YouTube direct messages, Google posted another article describing the process:
Go to this Google Takeout page.
In the YouTube section, select "All data included" to choose which data you want to keep.
Deselect all data except "Chats".
Select Next step.
Select “Creative archive”.
You’ll receive an email when your archive is ready.
Download all of your chats as a .zip file.
This functionality is the latest in a string of features and services that have been Killed by Google.