When a plan is retired from the catalog, are customers who are currently subscribed to that plan affected?

No, when a change is made to a plan in the catalog, such as a plan being retired, the change will not affect customers currently subscribed to that plan. In the case of a plan being retired from the catalog, the plan will simply no longer be available to new subscribers. In the case of a general plan modification, only new subscribers going forward will be affected by the change.