This enhancement is part of the May 15, 2021 release. 

Alert Center and News Flash item links from emails, SMS, and social media can now continue to direct to their content after items have expired or been archived.

This can be enabled for all categories in Alert Center, News Flash, or both by the CivicPlus Support Team.

If enabled, links to all of the module's expired or archived items will automatically be redirected to the most recent publicly visible archived version of the item. If the item's archived versions are all publicly hidden, links to it will continue to direct to 404 as they do now.

Resource 

News Flash 
Alert Center