Would it be reasonable to consider allowing indexing software to accept a question mark for the last digit of a year? To me it seems like 169? might be more helpful than <BLANK> for someone searching indexed records for an ancestor.
Have you tried it?
I believe I was able successfully to do that recently on Year Field in an entry. Whether you can or cannot might be project-specific. I used to think it was never valid. Maybe times have changed.
I used the ? yesterday on a year entry and the batch submitted. I don't actually remember which project I was working on.