How to delete a content type field without deleting the whole view?
As per the following issue registered on Drupal.org, that is what  happening with D8 views hence you are observing that phenomenon. While in drupal 7 the view  gets automatically updated. You  can find the registered issue with drupal 8 core on  drupal.org at following link. 

When deleting a content type field, users do not realize the related View also is deleted

That issue is in review phase, hence probably significant work would have been done on that issue. You need to refer that.

If you read the entire discussion there, currently they are working on the idea of Comment #59. suggested by @dawehner which is stated/pasted below,


  Here is alternative approach:
  
  
  Don't delete existing views
  Disable existing views and tell people which ones got disabled
  Then the user can go back to this view, fix missing fields and call it a day