Instead of creating a warning for every package manager out there, Perhaps a welcome message in the terminal would be a better solution? its just a matter of editing the ~/.bashrc script, and add an echo command at the beginning of the script:

# ~/.bashrc: executed by bash(1) for non-login shells.
# see /usr/share/doc/bash/examples/startup-files (in the package bash-doc)
# for examples

echo "Welcome to Manjaro GNOME 21.1.6."
echo -e "To install a package, type \033[1;32m\"sudo pacman -Syu <package name>\"\033[1m"
\""

The end result should be able to get the point across. Opening a new terminal window shows the following: