This has been posted in the Announcements section since September 2018:
There were several threads at the time that had Plex staff responding (including Elan) but I don’t know if they still exist or not.
As an aside, I’ve got Plex Media Server 1.18.0.1944 installed and WebTools still works for me although I don’t use playlists.
EDIT: The switch in server versions probably exposed the new interface and they moved Plugins. You can see the settings for Plug-in’s by clicking the Settings icon in the upper right hand corner, then clear at the bottom Plugins is under Manage.