In the past I could Edit an item (Movie or TV Show) and “tag” it with the name of a user on my Plex server. Doing that would make the item appear in that User’s library.
That functionality is no longer in the application.
How do I set the permissions on an item so that it is accessible to other users?
There’s never been a feature that tagging an item with a user’s name was automatically granting them access (at least not since I’m using Plex). You can assign sharing labels and use them when restricting/configuring a user’s library access (no matter if they’re in line with user names or not).