Libre 6.1.6 not optimized for my macOS 10.14.5?
Strange because afaik macOS throws this message for 32-bit applications while LibreOffice 6.1.6 for macOS is only available as 64-bit package and hence this message it shouldn’t appear. Are you sure that the messages wasn’t only thrown while install of 6.1.6 deinstalled an older 5.x 32-bit version (and you selected “delete previous version”). In fact I didn’t see this message on my test system.