The extension CategoryCheck reminds users to assign a category to a page before saving it.
Using the category check
To use this feature, it must first be enabled for individual namespaces in the Namespace manager
When saving a page in a namespace that has the category check enabled, the user is reminded to assign a category before saving. This message only appears if the page does not have at least one category assigned. The page can still be saved without assigning a category, but the user must explicitly check the "Ignore" checkbox.