Send old Simplenews to newly registered users?
I also found a brute way.

Set newsletter sending status to 0 which will allow a resend to the entire userbase.

Install https://www.drupal.org/project/simplenews_filter

Select only the users that were missed the last time via Views, and send.

(One could also select to send all and delete the subscriber ids from the spool table where you dont want to send)