Hi, it is not very clear how to request the addition of an extension to the Beta Cluster. Said that...
This is a request to add https://www.mediawiki.org/wiki/Extension:Newsletter aka MediaWiki-extensions-Newsletter
Currently the extension is functional, and we keep working polishing it. It is available in http://newsletter-test.wmflabs.org/ only, and we have requested T115095: Security review of Newsletter extension. A logical next step is to have this extension available for testing in the Beta Cluster.