Old Ubuntu use to have the /etc/acpi/sleep.sh etc. power management
scripts and the pm-suspend command that you’d call to suspsend the
computer. But, how do you suspend the computer in modern Ubuntu?
Easy, just use systemctl:
sudo systemctl suspend
Please note that the pm-suspend command is still available and is
part of Debian core, but the new way appears to have become more
popular, and in some distributions (i.e. Raspbian) the classic Debian
commands may even be broken in the default configuration.