In Ubuntu 20.04 is it possible to show only custom menu entries in grub?

From https://help.ubuntu.com/community/Grub2/CustomMenus it states the following to only use the custom menus in GRUB:

Using Only a Custom Menu

The following is one example of a way to use only a custom menu and not receive any automatic menuentry updates:

Copy/paste the entire contents of your /boot/grub/grub.cfg file into the /etc/grub.d/40_custom file, below the existing header lines.

    #!/bin/sh

    exec tail -n +3 $0

    # This file provides an easy way to add custom menu entries. Simply type the

    # menu entries you want to add after this comment. Be careful not to change

    # the 'exec tail' line above.

Remove everything above the first menuentry except the existing header lines above.

Remove all but the following files from the /etc/grub.d folder:

    00_header, 05_debian_theme, 40_custom and README.

    Alternatively, you may keep other files in the /etc/grub.d folder if you make them unexecutable. 
Edit, add, or remove menuentries as desired, then save the file.

Run update-grub as root to apply the changes!

Hope this helps!