How to set power button to shutdown instead of suspend?

That's caused by the latest gnome-settings-daemon updates...
There is no such option in power settings because it was removed by the GNOME devs (the shutdown/power off action is considered "too destructive").
Bottom line: you can no longer power off your laptop by pressing the power off button.

---

You could however add a new dconf/gsettings option (i.e.shutdown) to the settings daemon power plugin if you're willing to patch and rebuild gnome-settings-daemon:

--- gnome-settings-daemon-3.18.2/data/gsd-enums.h   2015-11-10 09:07:12.000000000 -0500
+++ gnome-settings-daemon-3.18.2/data/gsd-enums.h   2015-11-11 18:43:43.240794875 -0500
@@ -114,7 +114,8 @@
 {
   GSD_POWER_BUTTON_ACTION_NOTHING,
   GSD_POWER_BUTTON_ACTION_SUSPEND,
-  GSD_POWER_BUTTON_ACTION_HIBERNATE
+  GSD_POWER_BUTTON_ACTION_HIBERNATE,
+  GSD_POWER_BUTTON_ACTION_SHUTDOWN
 } GsdPowerButtonActionType;

 typedef enum
--- gnome-settings-daemon-3.18.2/plugins/media-keys/gsd-media-keys-manager.c    2015-11-10 09:07:12.000000000 -0500
+++ gnome-settings-daemon-3.18.2/plugins/media-keys/gsd-media-keys-manager.c    2015-11-11 18:47:52.388602012 -0500
@@ -1849,6 +1849,9 @@

         action_type = g_settings_get_enum (manager->priv->power_settings, "power-button-action");
         switch (action_type) {
+        case GSD_POWER_BUTTON_ACTION_SHUTDOWN:
+                do_config_power_action (manager, GSD_POWER_ACTION_SHUTDOWN, in_lock_screen);
+                break;
         case GSD_POWER_BUTTON_ACTION_SUSPEND:
                 do_config_power_action (manager, GSD_POWER_ACTION_SUSPEND, in_lock_screen);
                 break;

Once you install the patched version, a new shutdown option will be available in dconf-editor under org > gnome > settings-daemon > plugins > power > power-button-action:

so select that to shutdown via power button or, if you prefer CLI, run in terminal:

gsettings set org.gnome.settings-daemon.plugins.power power-button-action shutdown

Sure, for the above to work you also need the right settings in /etc/systemd/logind.conf:

HandlePowerKey=poweroff
PowerKeyIgnoreInhibited=yes

---

Keep in mind that pressing the power button will shutdown your system without any warning.

As @don_crissti explained, the situation is silly because it really isn't possible to just shutdown without confirmation using the normal settings.

However, there's a workaround: if the chassis type of the machine is set to "vm" (virtual machine), the old immediate shutdown behavior will take over any settings. Simply enter as root:

hostnamectl set-chassis vm

and reboot once. I don't know of any side effects of this settings.

In Ubuntu 18.04 or any similar Linux variants with acpi (if not you can probably install), make a file called /etc/acpi/events/power with

sudo nano /etc/acpi/events/power

and put

event=button/power
action=/sbin/poweroff

inside the file, close it, then

sudo service acpid restart

This is probably the simplest solution that definitely works.