Category:Undelete in 2090
Note: If you are basing the date on "70 years after the death of the author", note that according to the legal definition copyright always expires on 31st December of the calendar year of death. So, the file becomes copyright free on 1st January of the following year. The correct year category to use is thus Year of Death + 70 + 1.
Important: You must include the deletion request subpage in the category, not the image itself, as a deleted file will not show up in the category. Alternatively, just add the name of the file to the category.