Apologies, I have not been clear enough again.
There used to be a little 'play' arrow that appeared beneath a photo. I clicked that, and if I was in an album, a slideshow would start, limited to that album; if I was in my entire library, a slideshow would start including the entire library. It would shuffle or go sequentially depending on the shuffle toggle in settings.
That little arrow is what I am missing--or the functionality it provided.
I can see options to select individual photos, and then play a slideshow from those photos, but I have 10,000 photos in my library, and need a way to get to the slideshow without selecting 10,000 images manually first. I do not see any way to start a slideshow without selecting all the images first, or an option to do a 'global' select of an entire album or--my usual habit--the entire library at once.