PSA: Somehow, without action from me, hyberfile.sys got turned on

This is just a PSA where my circa 2009 desktop works just fine on Windows
10 but today, it was out of memory even though I had about 5GB left.

Huh? 
Where did 5GB of data suddenly go? 
  C:\> wmic logicaldisk get name,freespace,size
  C:\> vssadmin list shadowstorage
       
I checked all the usual suspects (which is a long list I maintain) such as:
  C:\> dism /online /cleanup-image /analyzecomponentstore
  Win+R > cleanmgr > Clean Up System Files > OK
  C:\> del /s /q C:\Windows\Prefetch\*
  C:\> del %LOCALAPPDATA%\CrashDumps\*
  etc.

But it wasn't any of the usual suspects!
It was hyberfile.sys!

WTF?

Sleep is off. Fast startup is off. Everything related to power is off.
Point being that I haven't turned on hibernation in more than a decade.
 Q: How the heck did hibernation get turned on without me doing it?
 A: I don't know the answer.

But the solution was simple once I saw that hiberfile.sys was the culprit:
 C:> powercfg /h off

Note: Obviously I had turned that off years ago, so "something" in the
recent days turned hiberfile.sys back on. But what? I do not know.
-- 
Note to help others, Windows 10 allows for a "reduced" hibernation file
that only supports Fast Startup without allowing full system hibernation:  
  C:\> powercfg /h /type reduced