Subscriptions in GraphQL are the special requests where you ask the server once about the changes in specific Data related to the query and the server will keep polling the client with changes.
We need subscriptions in case of systems like comments or chat and it is not for Login Systems
I do not get it, why are you validating on Login/Authentication system. You don't need to.
Whatever the form fields are, do a regular sanity check on the fields on the client side and send to the server; the server will figure out whether to let the user in or not.