How do I copy one Keynote slide to another Keynote presentation?

Open both Keynote files.

Click the slide that you want to copy.

Click the Keynote window that you want the new slide copied to.

Highlight a slide, where you want the copied slide to appear.

Then paste cmd + v.

Then you are done.

Maybe things have changed since 2015, but when I copy and paste this way, all the content from the original slide gets grouped together as one piece, and cannot be ungrouped in the target presentation. Is there any way to really insert a slide ‘as is’ into another presentation?

Ok. Well you shouldn't need to copy the content of the slide. Although it's possible to do it that way, only a bit more complicated. It's important that you have selected the slide that you need to copy, not any of the content of the slide.

Here's how I do it, without touching the content.

OK, I’m able to answer my own question: drag and drop from the old to the new presentation works. Not ideal for those of us who prefer the keyboard to the mouse, but it works.