Actual result:
Normal-looking edit window appears

I did not see a "Normal-looking edit window " when I clicked https://en.wikipedia.org/wiki/MediaWiki:Gadget-XFDcloser-core?action=edit
In fact there's no edit window at all, if there's one that would be a bug per T249978#6048312

...and the only notice is an edit notice is the standard notice to admins about editing in the mediawiki namespace

That's not editnotice. It's a standard core permission error message for lacking permission to create the page in NS_MEDIAWIKI. Perhaps over time some wikis may have over-modified it which makes it to start looking like something else.

This is the error message as provided by core unmodified. On enwiki even though it's modified, there's no edit window. Can you explain how can we reproduce the normal-looking edit window that you saw?

I use the new wikitext editor, and it proceeds to open the edit window --~~~~

Then that's a VE bug, but the bug report implied otherwise.

Unfortunately the Editing team can not prioritize this at this time.

Actual result:
Normal-looking edit window appears, and the only notice is an edit notice is the standard notice to admins about editing in the mediawiki namespace

This might have been already fixed/improved? Indeed the editor opens as normal, which is confusing, but there is also a clear message that the page can't be edited (and you indeed can't type in the editor).