The -n switch still works with my copy of LibO 3.5 downloaded from the project web site.
It makes any ODF document behave as if it were a template. You don't need it if the file is a template already (*.ot?).
Likewise, the -o switch opens a template for editing just like an ordinary document.
Please, edit this topic's initial post and add "[Solved]" to the subject line if your problem has been solved.
Ubuntu 18.04 with LibreOffice 6.0, latest OpenOffice and LibreOffice
In LO3.5 in Kubuntu - the developers have moved the default coustomization folder from ~/.libreoffice, to ~/.config/libreoffice. Ones I've moved my ~/.libreoffice - to ~/.config libreoffice - all of my customizations got restored and work fine.