Unhiding people from People & Pets album in Photos for MacOS

Hidden can mean two different things. I think you mean that you chose "Hide this Person" in the People and Pets section

which doesn't cause the person's pictures to shift to the Hidden view. Rather, the People album disappears from the People and Pets section-- is this right? If so, then the People album still exists-- it just doesn't appear in People.

You can still find the picture and the People album. I would search for Altha, for instance, and I'd find a bunch of pictures with Altha in them. Then, on a picture with Altha in it, I hit the space bar (or double click) to see a full screen preview. Then if I push the picture up, I get extra stuff below, including a list of the albums for each identified person in the picture.

There's Altha and her husband Orville. They both have people albums, but Alta's is hidden. If I double click Altha, then her People album opens. Push up to see the bottom stuff

And you get a choice to "Add Altha to People & Pets"

Does this work for you?