question about client/server version mismatches

Hi,

macos-10.11.6
postgresql-9.5.16 (server)
postgresql-9.6.12 (client)

I've just rebooted my macos laptop for the first time in months
and have started having a problem loading database backups there.
I get this error output:

  pg_restore: [archiver (db)] Error while INITIALIZING:
  pg_restore: [archiver (db)] could not execute query: ERROR:  unrecognized configuration parameter
"idle_in_transaction_session_timeout"
      Command was: SET idle_in_transaction_session_timeout = 0;

It seems that I was using 9.6 clients and a 9.5 server.
I'm not sure what the situation was before the reboot.
The server was probably 9.6.12 and all was well.
I never got around to fully upgrading the server
(or the old version wouldn't still be there).

Loading a 9.5 backup using the 9.6 pg_restore lead to
the above error. Changing to 9.5 clients fixed it.

Since the backup itself was from a 9.5.12 server, it
seems that the 9.6 parameter, idle_in_transaction_session_timeout,
must have been set by the 9.6 client even though it was
connected to a 9.5 server. Is that expected behaviour?

Do the clients know when each configuration parameter
was introduced and only use them when connected to servers
where they mean something? Or am I just misunderstaing
what's happening? I would have thought a new client
would be able to work with an old but supported server.

It's not a big deal. I'll get around to completing the
upgrade and it'll be fine again but I'm curious.

cheers,
raf