How to exclude views fields from display when a node is flagged
General approach:

Use the Rules module (which does not yet have a stable release for Drupal 8, but does have the capability in its current state to implement the recipe below) to detect the content is flagged, then do a Rules action which will result in those fields being hidden.

Here is one specific example, but there are many ways, and likely simpler ways to do it than my recipe. I wanted to give you an example that doesn't require coding or extra modules besides the Rules module:

Drupal automatically includes a body class with the content type of the node. So make a rule that will evaluate that a node has been flagged, and if it is, change that node's content-type via the action "Data => set a data value." Now you can easily create the css to hide the fields that you want hidden by using the body class for the content type assigned flagged nodes.

Make sure the two content types use the same fields and have all the same settings for those fields.

Also make sure to set up a second rule which does the opposite so that the content type is returned to the original when the flag is removed.