I’m having a play around with widgets and have created a stack with Mail, Calendar and OmniFocus.
Now I have a new widget show up which is Batteries, but I didn’t put it there. It seems to me that the widget is being added by Siri Suggestions. The name of the app is grey (others I added are white) and I can turn off suggestions for that app if I wish.
I too have recently encountered this frustration. I searched all the Siri options but there is none to turn off suggestions for widgets. I wish Apple would not force these things upon us but instead give us the option to opt in or opt out.
Long press on the stack to get the menu up, then click on Edit Stack. At the bottom of the screen is the option to allow/disallow Siri suggestions and another to allow/disallow auto-rotate.