T110645: Interwiki support for Newsletter extension implies several pieces. Probably the first one (and simplest?) to be addressed is the possibility to use interwiki links when announcing new issues.
Currently new issues can only point to wiki pages in the same wiki where the Newsletter extension is enabled. It is easy to imagine newsletters where having the freedom to link to pages in Meta, Commons, phabricator tasks via phab:...
VisualEditor already offers this when linking to a page. The deal is that when defining an interwiki link, no type-ahead or verification of such link is offered. That's fine, and much better compared to the limitation of posting in a single wiki. Maybe we could simply copy that behavior?
It is worth noting that VisualEditor also allows to link to a non-exiting page (Newsletter does not at the moment). The link appears in red, but it can be used. In the case of Newsletter it is probably better to keep the requirement for an existing page in the same wiki, unless an interwiki link is used. However, if the price of interwiki links would be to remove this requirement... so be it?