Impress has a menu item File->Versions…, where I can create versions and where I also see a button “Compare”. However, that button is deactivated (shaded gray). How can I compare versions?
This is not implemented.\
The versions comparison is implemented as text comparison in e.g. Writer. As Impress is essentially graphic-based application (in its core, it’s Draw), and we haven’t graphic objects comparison, so the only way to compare versions is to inspect them visually s(l)ide-by-s(l)ide.