Adding a commented line to a file should not have any effect, no matter what the file is or what the comment says. It should just be only a comment.
If you change a line which is actually doing something to be a comment instead (by adding a “#” in front of the line, or whatever the commenting syntax is for the file you are working in), that will have an effect because you are effectively removing that line from the file. But in your example here, you have left the line which is having an effect alone, and added a comment to a separate file so no change was made.
You can override the line in the main file if you wish by providing a different setting in the drop-in file. For example, if you want Wheel to be able to sudo as root but a password is required, you can add the line in your drop-in like this:
%wheel ALL=(ALL:ALL) ALL
This will override the NOPASSWD version in the main sudoers file, because the options are read and applied in order and the drop-ins come after the main file.
But in this case I would say it would be more logical to edit the main file with sudo visudo and properly remove the unwanted amendment, instead of having a modification for a modification which provides what is essentially the default behavior.
It sounds like you may have set this up correctlyt, but just to double-check: this directory should be /etc/systemd/logind.conf.d/.
Unlike the sudoers drop-in, the drop-in here should be named with .conf. powerbutton_suspend.conf should be fine. It shouldn’t be a hidden file (with the “.” in front), I’m not sure if that will break it but it certainly is not needed.
[unit] is not right, it should say [Login]. See the reference written in the comments of /etc/systemd/logind.conf:
# This file is part of systemd.
#
# systemd is free software; you can redistribute it and/or modify it under the
# terms of the GNU Lesser General Public License as published by the Free
# Software Foundation; either version 2.1 of the License, or (at your option)
# any later version.
#
# Entries in this file show the compile time defaults. Local configuration
# should be created by either modifying this file (or a copy of it placed in
# /etc/ if the original file is shipped in /usr/), or by creating "drop-ins" in
# the /etc/systemd/logind.conf.d/ directory. The latter is generally
# recommended. Defaults can be restored by simply deleting the main
# configuration file and all drop-ins located in /etc/.
#
# Use 'systemd-analyze cat-config systemd/logind.conf' to display the full config.
#
# See logind.conf(5) for details.
[Login]
#NAutoVTs=6
#ReserveVT=6
#KillUserProcesses=no
#KillOnlyUsers=
#KillExcludeUsers=root
#InhibitDelayMaxSec=5
#UserStopDelaySec=10
#HandlePowerKey=poweroff
#HandlePowerKeyLongPress=ignore
#HandleRebootKey=reboot
#HandleRebootKeyLongPress=poweroff
#HandleSuspendKey=suspend
#HandleSuspendKeyLongPress=hibernate
#HandleHibernateKey=hibernate
#HandleHibernateKeyLongPress=ignore
#HandleLidSwitch=suspend
#HandleLidSwitchExternalPower=suspend
#HandleLidSwitchDocked=ignore
#PowerKeyIgnoreInhibited=no
#SuspendKeyIgnoreInhibited=no
#HibernateKeyIgnoreInhibited=no
#LidSwitchIgnoreInhibited=yes
#RebootKeyIgnoreInhibited=no
#HoldoffTimeoutSec=30s
#IdleAction=ignore
#IdleActionSec=30min
#RuntimeDirectorySize=10%
#RuntimeDirectoryInodesMax=
#RemoveIPC=yes
#InhibitorsMax=8192
#SessionsMax=8192
#StopIdleSessionSec=infinity
You can read more about the options in this file with man logind.conf.
So you drop-in file should be like this:
[Login]
HandlePowerKey=suspend
You will have to log out and back in or reboot after getting this set up.
If you are using a display manager to log in, typically logind.conf will “just work”. If you log in from a TTY or have a home-rolled solution for that you may need to manually send an idle hint to get it working.