Ubuntu – Ubuntu 20.04 LTS can’t Hide grub menu

Normally if you had only one OS running the option GRUB_TIMEOUT=0 would work. If GRUB 2's os-prober identifies additional operating systems while running the /etc/grub.d/30_os-prober script the hidden menu timeout feature is disabled by conditional statements. This also disables the ability to use the Shift key to display the menu during boot. Users with multiple operating systems wishing to hide the menu can find script edits on various forums which will allow them to add a hidden timeout feature to the boot sequence.

Check this answer if it helps you.

In /etc/default/grub set:

GRUB_HIDDEN_TIMEOUT="1"
GRUB_HIDDEN_TIMEOUT_QUIET="true"
#GRUB_TIMEOUT="1"

That will allow you to have a very short time interval of 1 second to still press Shift while booting to get the advanced menu, while not running into the 10-second problem in

if [ "${timeout}" = 0 ]; then
set timeout=10
fi

in /boot/grub/grub.cfg.

(Meaning, "0" is allowed, but just gets set to "10" in a deeper level of the config to protect users from themselves)