atomicpoet, 
🚨 NEW MASTODON FEATURE 🚨
When you receive a private message in your notifications, it is now clearly labelled "@ Private mention".
This improves usability vastly as it means fewer people will mistake public mentions for private mentions.
While private messages are not exactly "private" in the sense they are encrypted, they are still more private than public 😉
This feature's not yet available to everyone, but it's usable on the web version of mastodon.social.
See screenshot.