[Copying slides from one presentation to another]

Thank you for this information;

          so sad that it was removed, & now hoping that you might know
which LO edition last had this option so I might change to that one.

       And for all those in programming - will LO be putting this option
back into their program?

Menu Slide > Insert Slide from File

That may work for inserting only one slide, but what Anne wants is to
insert an entire Impress presentation, not just a slide or two. This can
be done with LO 5.1.6.3 (which is what I use), but it apparently is
missing in later versions. I can understand and appreciate Anne's
frustration that a former functionality seems to have been deliberately
removed from the program (unless it has just been moved to another
location on the menu, which nobody seems to have been able to find).

Yes, a person can copy and paste select slides, but that is hardly a
satisfactory answer when she used to be able to insert an entire file
and can no longer do so.

Virgil

You can select one or all the slides from a presentation. It is the
feature which was available until LibreOffice 5.2 under Insert > File
which has been moved to the Slide menu since LibreOffice 5.3.

As an alternative you can Select All slides in the Slide Sorter view,
Copy them (Ctrl+C) and Paste them in the other presentation (Ctrl+V) in
the Slide Sorter view. They will be inserted after the selected slide.