Managing Hidden Photos in Photos 5 on Catalina
The different flavors of "Hiding" - Keeping Items Secret vs. Managing them
The "Hidden" Feature has been changed between the versions of Photos.
In Photos 1 and 2 (macOS 10.10 to MacOS 10.12, iOS 9 to iOS 10) a hidden item has only been hidden from the main Photos view - the Moments, Collections, Years. But hidden items would appear in any albums. The purpose of "Hiding" has been a temporary hiding, just to remove some items from the moments without having to delete them from the library. We could organise the hidden photos in albums and work with them. This implementation of "Hiding" has been similar to the "Reject" rating in Aperture. It did not help to keep private photos secret, but helped to manage new photos.
The only draw-back has been, that hidden photos did no longer show the locations in the Info, and the country of the hidden Photos would not appear in the Year view. We had to unhide an item to see the location on the map.
Since Photos 3 the hidden items are hidden everywhere. This helps to keep photos out of sight. They are hidden from all views and appear only in the Hidden album or in smart album with the rule "Photo is Hidden".
It is now easier to keep photos out of sight, but more difficult to use the hiding feature to reject photos temporarily, because the Hidden Album has just a flat structure, sorted by the capture date. And we cannot see them in the context of the photos, that are not hidden. "Show in Moment" does not work for hidden photos.
Managing Hidden Photos in Photos 5:
In Photos 4 and Photos 5 the Hidden Album can be filtered using the "Showing" filter. Using the filter, we can select subsets of the hidden photos by filtering for edited items, Favorites, photos or videos, or keywords or a combination of these criteria (How to use the Filters in Photos 4.0 - Apple Community).
To work efficiently with hidden photos and videos, it will help a lot to tag our photos with keywords to find them quickly in the Hidden album.
To make the hidden photos visible in albums, we can use smart albums (File > New Smart Album).
Combine the rule for the smart album with "Match All" and add a rule "Photo is hidden".
For example to see the currently hidden photos in my album "Restaurieren", I am using this smart album:
Hidden Photos and Locations:
A nice new feature in Photos 5 in Catalina is, that hidden Photos are now showing the location in the Info. We do no longer have to use "Image > Unhide Photo" just to see the location.
In Photos 1 to Photo 4: The Info panel of a hidden Photo does not have a location field at all. We can neither see the location nor add locations too hidden photos. Here is an example, of what I am seeing in macOS 10.14.6 Mojave:
But in macOS 10.15.4 Catalina the hidden photo is showing the name of the location and the tiny map with the pin. We can tag the hidden photos like any photos when working with them in the Hidden album or in in a smart album.