Patroni : How to handle a replica which has been disconnected from primary for long time?
I would assume you are monitoring your DB cluster health, so a missing replica would pop up very soon.  Also, it is a must to monitor disk space (running out of it might bring you into a situation that is not very easy to solve), so that would also catch this (later than sooner, usually).  

Once you discover you have a replica that fell back, you have to investigate why it did so, and fix it - or remove the host from Patroni altogether.  If under disk space pressure, remove the replication slot to free up WAL space.  In a cloud setup, often simply terminating the host will solve all this by bringing up a new host.  In any case, once you have a functioning host, you might have to reinit the Patroni node.

On the other hand, I'm afraid currently there is no mechanism for fencing off replicas that doesn't appear to come back (be it any actual implementation from removing the replication slot to anything more complex than that).