I think we don't want to make assumptions about the remote end being the same after the upgrade and we let users reactivate the subscriptions in a suitable way. See [1] (Start reading from "..When dumping logical replication subscriptions..") In your case, if you wouldn't have allowed new tables in the publication then a simple Alter Subscription <sub_name> Refresh Publication with (copy_data = false) would have served the purpose.
I understand that this is not related with version 14, pg_upgrade would do the same steps on previous versions too.
Additionally it would be interesting to document that pg_upgrade does not upgrade completely if the server is a subscriber of logical replication, so it´ll have pre and post steps to do if the server has this kind of replication.
