On Drupal 8, I created a new content type, MyContentType, and accepted all the defaults.
I noticed that Create new revision under Publishing options was checked but slightly grayed out and I could not adjust the value, even though I was logged in as user 1:
I don't know why I can't uncheck it, but this is fine because I always want to save a new revision every time the node is saved.
However, to test this, I then created a new node of MyContentType and saved it. Then I edited the saved node-- and the Create new revision checkbox is unchecked!
So Create new revision is permanently enabled on the content type edit page, but on the actual edit pages for that content type, the revision box is not checked by default.
What is causing this bizarre behavior?
In Drupal 8.2, create new revision became checked by default, but it says nothing about making it "mandatory by default".