Missing help takes one to page which points to Bugzilla and mailing lists
There is a "Help" button in the PDF import window. I clicked it. It took me here:
https://docs.gimp.org/2.10/en/help-missing.html
It links to Bugzilla which is another 404:
https://bugzilla.gnome.org/enter_bug.cgi?product=GIMP-manual