I think the reason is that only the archiver process loads the library, so the GUC isn't registered at startup like you'd normally see with shared_preload_libraries. IIUC the server will still create a placeholder GUC during startup for custom parameters, which is why it shows up for SHOW commands.
Thanks for the quick answer !
That's a little surprising at first but I understand better now.
Will there be a facility to check archive_library gucs later on ? It might come in handy with more
