I made one of my repositories into my OpenRSS, so I am using RSS for the short brief or content delivery instead of the newsletter.
But some people like newsletters and for them, I left my TinyLetter active.
I don't want to write twice the same, so I connected both things - GitHub Issue and TinyLetter email with Zapier.
I do not plan to send numerous newsletters, so Zapier is just fine. Otherwise, you will need something more profound.
A canned Gmail response didn't do the trick, as TinyLetter has unique reply emails and it only messed things up.
I am sure there is a better thing to do this, but this is by far the easiest way.
TinyLetter gives the ability to send a post with a secret email and after that it sends an additional email asking you whether you want to publish.
I created two "zaps" in Zapier, one sends the content of a GitHub Issue to TinyLetter through email, and another one sends a confirmation reply.
Important thing is to set the resulting action to REPLY email, not SEND email, as it keeps a subject line.
Second, each reply-to email address for TinyLetter post confirmation is different, so that is one thing that should be set with a template in a zap.
In this way, I do not need to waste additional effort to publish the same thing in two different places, and neither I need to touch anything.
Mail will do all the things on auto.