The Newsletter extension has been deployed to test.wiki!!!
https://test.wikipedia.org/wiki/Special:Newsletters
One problem for testing this extension is that only administrators can create newsletters. This is intended behavior as agreed in prior discussions but... it doesn't help (me) testing this extension here and now. I will request admin rights just for this.
Anyway, this is a task to share any feedback about this last testing phase before deployment in MediaWiki.org.