How do I delete an entire slideshow in Photos?

Right-Click or Control-Click and choose delete when selected in the sidebar.

If you highlight the slideshow thumbnail in the main window, then the delete key or command-delete will work, or choose delete under Image in the menu.

thanks, helpful

No one here knows "why" anything - we are users just like you and have no knowledge of "why" Apple does anything - you can ask them or tell them what you would like - https://www.apple.com/feedback/

LN

Thanks ... very useful. But why does the photos program not make this obvious and/or include it in the edit menu? But thanks for your help. Much appreciated. Peter (New Zealand)