I just went through 2 hours of reading and trying different methods. Hopefully someone like me finds this thread easily by looking up something like “Plex prevents windows from going into sleep” or “Windows 10 is not going to sleep”.
After trying what is on this thread and many, many others, I noticed that the suggested changes to the settings no longer work for Windows to actually go to sleep.
What did it for me, and friends that had the same issue, was going to:
Control Panel > Power Options (in icons view, not category; click category in blue to change) > Change Plan Settings (in blue near whatever Preferred Plan is selected) > Change Advanced Power Settings > + Multimedia Settings > + When Sharing Media > Setting: Allow the computer to sleep.
This seems to override the Plex request to never go to sleep:
System Availability Requests:System Required Request
The program has made a request to prevent the system from automatically entering sleep.
Requesting Process \Device\HarddiskVolume6\Program Files (x86)\Plex\Plex Media
Hope this helps!