FYI
After Sept. 18, you’ll no longer be able to send or receive direct messages on YouTube as we focus our efforts on public convo tools (comments, posts, etc).
You can still share what you’re watching w/ others via the Share icon below videos.
More→ https://yt.be/help/9Hns
You've removed this feature back in 2013, don't you remember? An inbox that was actually used by a lot of people regularly, but you really had to replace it with the shitty Google+ one, and then another one that was only accessible through a smartphone. Of course no one used it.