This is a virgin install of 20.04, not upgrading from 18.04 and previous versions. I tried to add an item to Other using MenuLibre. Launcher tested OK, saved, etc., yet Whisker menu still not showing "Other" category. If I go back to MenuLibre it does not even show my addition. Yet, in my $HOME all changes and the launcher file is created. I researched the subject, consulted answer here "How to reset MenuLibre"; purged, deleted related files, reinstalled MenuLibre, but nothing helps. This really baffles me.
Further notes a few days later: After various attempts finally I have the "Other" category in Whisker menu with my custom added menu items (launchers). However, I had to do it "under the hood" and not with MenuLibre. Here is how:
- deleted 
~/.config/menus/applications-mergedand~/.config/menus/gnome-applications-mergedfolders - created my launchers (.desktop files) in 
~/.local/share/applications/with Mousepad, according to relevant specs - edited 
~/.config/menus/xfce-applications.menuXML file with Mousepad in the<Name>Other</Name>section within<layout>part by adding there<Filename>MyOwnLauncher.desktop</Filename>lines of my own Other menu items. 
This seems to work. Maybe tedious, but not as frustrating as MenuLibre. I even created a new user to see if MenuLibre would work there - but not really. Same thing, Other is not shown. The new menu item in the other category is in fact created in ~/.config/menus/applications-merged as .desktop files, but invisible to Whisker menu.
Could this be a bug in MenuLibre for the 20.04 xUbuntu version?