Gift Registry feature is a tool to assist the customer in purchasing gifts for someone registered and the event date is just a date to notify buyers regarding the event. Here, all the gift registries will remain as it is even after the expiration of the event date. Also, all the products available in someone’s registry will also remain in the registry.
For checking this feature refer Gift registry demo store.
As example below, the event date is 30.03.2020 whereas the screenshot is taken on 15.04.2020.