After my device woke up from hibernation yesterday, I've been missing the 'Shutdown', 'Reboot' etc. buttons. Basically, only 'Logout' and 'Lock' are left. These were missing even in SDDM. So I checked my packages and apparently ConsoleKit2 got removed... 'polkit' too (maybe it was never installed, I'm not sure). I don't know how this happened. How can a system remove a package at its own whim? Anyway, that partially fixed the problem as I can now shutdown and reboot from the DM, once I logout. The workspace menu still only has the above mentioned two buttons.
Per this thread: https://forum.kde.org/viewtopic.php?t=107214 I tried inside terminal:

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qdbus org.kde.ksmserver /KSMServer canShutdown

and it returns 'false'. I am more-or-less a novice at kde plasma and dbus. Help would be appreciated, thank you.

Okay, apparently ConsoleKit2 and polkit have little if anything to do with this, because Shutdown and Reboot are seldom clickable even in the DM. After I have disabled them, nothing changed. Suspension is always greyed out and my laptop can't enter suspended mode even on inactivity. Can anyone just point me in any useful direction here? I'm completely lost.
The NetworkManager is acting up. I cannot connect through wi-fi to new SSID's and can't add new connection in the nm desktop applet; it does connect to old access points automatically and I can connect with wpa_supplicant. I have a feeling this is all tied together, that it's stemming from the same problem. I doubt it's permission related because it doesn't work even after adding myself to the root group and restarting. I have no idea where NetworkManager is reading the connections from. I thought adding a

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.nmconnection

file manually to

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/etc/NetworkManager/system-connections

would get read by the applet (plasma-nm), but it doesn't get listed even after restarting the system.

It might seem like these two are completely disconnected, but I repeat, it feels like this is a DBus issue, or something. But then again, I'm a n00b.

I have managed to figure out the problem. Apparently it WAS a polkit/ConsoleKit2 issue. I have uninstalled ConsoleKit2 and installed elogind and the power buttons reappeared. The NetworkManager thing was fixed, too. It seems there are some compatibility issues with ConsoleKit2 and KDE (maybe someone could expand on this).
This problem probably occurred while switching from Cinnamon.

Additional info:
Running pkexec actually pushed me at the right direction as it gave the following error:

polkit-agent-helper-1: error response to PolicyKit daemon: GDBus.Error:org.freedesktop.PolicyKit1.Error.Failed: No session for cookie

Checking ~/.local/share/sddm/xorg-session.log, I found this:

polkit_agent_listener_register_with_options: assertion 'POLKIT_IS_SUBJECT (subject)' failed"Cannot register authentication agent!"Authentication agent result: false

I DuckDuckWent the above and stumbled upon this thread:
https://forums.gentoo.org/viewtopic-t-1084422.html
I guess it's important to note I use runit as init on Void Linux. I hope this thread will be of some help to someone.

Anyway, thanks to everybody who helped.