Actually I never noticed a field called "Release Date" in Get Info of previous versions, only Year. And there is no Release Date field shown in any of my music Get Info. If I try to access Get Info for albums, it asks me if I want to "edit information for multiple items." If I click on Edit Items, there is still no Release Date field - just Year.
However, not one of the Release Dates are correct in any of the books, i.e. those imported into iTunes both before and after the latest "upgrade." AND the Release Date column is blank for them all. So what is it for, anyway?
It is a pity that, if it isn't editable, it should be able to be removed or added depending on the need.
Just found out another weirdness. In the Audiobook List Genre is no longer there. It is in the Playlists, thank heaven, but not in the master list where it really should be. It IS in Music, but not in books. How weird. I hope that is an oversight and will be fixed.
Thank you for your response.