Does macOS Sierra support multiple users simultaneously connecting to the same machine using screensharing?

The feature still exists, yes. With some experimentation, I managed to understand the trick in knowing how to invoke that system.

My goals were to get a "not mirrored" remote desktop session going, in case I ever needed to work remotely for any reason.

TL;DR - First "screen sharing" session is always mirrored; subsequent sessions will prompt

Here are the steps to reproduce (though, not quite 100%, I noticed; unsure why):

1. The "Server" needs at least 2 users who are able to use Screen Sharing.
2. Have your "Client" launch screen sharing with the "Dummy" user.
   • Finder -> "Go" menu -> Connect to server (Command+K)
   • Enter the credentials for dummy
   • This is the "first screen sharing" session, and will mirror the displays.
3. Have your "Client" establish a second connection the server using your "Main" user.
   • Screen Sharing (application's name) Menu -> Connection -> New...
   • Enter the credentials for main_user
   • Dialog box should prompt "Dummy is already logged in. Ask for permission to share the screen, or login as [main_user]?"
   • Choose the "login as [main_user]" option.
   • This creates a "secondary screen sharing" session which will NOT be mirrored; it will be a virtual session.
4. Close the "Dummy" Screen Sharing window.
5. Close the "Main" Screen Sharing window.
6. Have your "Client" launch screen sharing with the "Main" user.
   • Somehow, dummy will still have a session running and the system will once again prompt the "ask permission? or just log in as yourself?" questions.

macOS Sierra still supports multiple simultaneous connected users. In the example below Sierra is running in a VM and admin is logged in. user is a remote screen sharing session from the VM host machine (10.9.5) – an iMac – and root is a remote session from a MacBook Pro (10.11.6):