Can someone delete all pages with content model Newsletter on snapwikiwiki? This is a blocker for creating dumps when the extension is disabled. Someone also might want to look into giving better errors on Special:DataDump.
John pushed a commit to ManageWiki today that allows removing namespaces when extension is removed. I'll update ManageWiki and push my implementation to mw-config to resolve this later. All you'll then have to do is toggle the extension again, I think.
I'm still working on a fix to ManageWiki, which if needed can be a new task, but deleting just https://snapwiki.miraheze.org/wiki/Newsletter:Wiki_Notices should resolve this task in the meantime. Thanks!