With the latest versions of LibreOffice you have two different options
for inserting slides from one presentation into another one, both tested
and working on every platform.
A. You can open two different LibreOffice presentations at the same
time, and switch between them using the Window menu.
When you have two presentations open you can copy and paste slides
between them from the Slide Sorter view:
1. Select multiple slides with the mouse while keeping the Ctrl key pressed
2. Copying them by selecting Edit > Copy or by hitting Crtr+C
3. Switch to the other presentation from Window > Presentation Name
4. Past the slides after the selected slide from Edit > Paste or by
hitting Ctrl+V
B. You can insert multiple slides from another presentation into the
open presentation from Slide > Insert Slide from File...
1. Search and open the source presentation
2. Select any number of slides or the entire presentation
3. Click OK and wait for the process to complete
I have just copied/pasted according to option A and inserted according
to option B 60 slides of an 82 slide presentation into another 142 slide
presentation without any issue.
Although I don't see the need of reverting to an old LibreOffice version
to do the same thing in a different way, you can find old LibreOffice
releases here:
https://downloadarchive.documentfoundation.org/libreoffice/old/