Plex has not altogether ended support for plugins. Plex has only discontinued the use of accessing plugins through its in-app Plugin “Channels” directory. If a plugin—say the YouTube “channel” plugin—required the use of said directory, that plugin no longer works. The Audible Agent does not require that, and therefore is not affected by this change.
Now, with that being said, the current way to use the Audible Agent, assuming it is installed correctly, is to open your audiobook library’s settings (Mouse over library name → click 3 dots to right of library name → Manage Library → Edit… → Advanced) and change the agent for your audiobook library to the “Audiobooks” option.
The Audiobooks Agent should also appear in the Agents submenu in your server’s Settings menu.
