Splitting this request from T401274.

Editors should be able to unwatchlist pages that have been deleted. In practice we don't need to check that a page has actually been deleted, just that it doesn't currently exist.

This could be a filter that is grouped with the namespace filter (T405141), so that users can combine the filters and remove, for example, all Draft pages that are deleted. Combining deletion status and 'edited in X days' maybe doesn't make as much sense, so there might be some design work to figure out how to enable combining each of these filters in a way that makes sense, unless we adjust the wording and this apples a 'deleted more than X days ago' filter instead of looking at edits.

The setting proposed at T401274#11199182 would apply to this criterion (i.e. checking also if the discussion page exists)