How to have all the fields available in a display mode for content type D8 I'd like to be able to choose the date field or other fields that you can see in form display in 'custom display mode' for content types ?

How can I do that ?

Thank you.
You need to drag the items that you don't want to the disabled section. This works for both form display and front end display. 



For your custom display mode, you should be able to select from each display mode: