You have one Plex shared folder (the old DSM 6 one)
You have one PlexMediaServer shared folder (the new DSM 7 one)
When you were using DSM 6, the Plex shared folder would be recreated automatically
As of DSM 7, we no longer create Plex or user Plex.
In DSM 7, the PlexMediaServer shared folder will be recreated as well. This is how the package works.
In DSM 7, Synology required a name change. The new name I chose is “PlexMediaServer” because it is also the new Package Name (also a required change).
If you don’t have any metadata in the old “Plex” share;
specifically, you don’t have a Plex/Library/Application Support/Plex Media Server directory,
then it’s ok to remove the Plex shared folder (Control Panel - Shared folders) if you have no use for it.
There are two threads which detail what changed from DSM 6 → now.
The initial DSM 7 package
This final structural change (which I wanted in the first but couldn’t make it work right)