SUMMARY
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This is regression after upgrading to 5.24. KDE Plasma no longer follows the setting Power Management > "when asleep, hibernate after a period of inactivity". It should target the suspend-then-hibernate systemd unit, like it always did. I figured it out when I found my laptop after a full day of inactivity to resume quickly.

Something's really off here - initially the option was unchecked on every tab "On AC Power", "On battery", "On low battery". Systemsettings cannot save this option on "On AC Power" tab. When I reopen systemsettings, the option is unchecked again, but only on the first tab. It doesn't change the fact that when it goes sleep on AC unplugged, it still goes for regular sleep.
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STEPS TO REPRODUCE
It only works on Linux systems with hibernation set up properly.
1. Open Plasma System Settings
2. Go to the Power Management section
3. Select "When asleep, hibernate after a period of inactivity" on every tab. apply settings
4. Close laptop lid or click on sleep option in the application menu.
5. Leave the computer for few hours (that depends on some global system configuration, but I can't remember which one)

OBSERVED RESULT
The computer stays in a suspend mode even after 24+ hours.

EXPECTED RESULT
The computer should be suspended at first, but hibernated few hours later.

SOFTWARE/OS VERSIONS
Linux/KDE Plasma: 5.16.8-arch1-1
(available in About System)
KDE Plasma Version: 5.24.0
KDE Frameworks Version: 5.99.0
Qt Version: 5.15.2

ADDITIONAL INFORMATION
I use the option since it's available in Plasma and it feels like breaking it on new release and then fixing afterwards is sort of a tradition once in a  while.

The whole Power Management section is really bad. It's so bad that I hope it never broke so that I didn't have to have to open and look at it again. It's not a part of the report, but it would be really nice to see general rework of the section taken into consideration.