Why can't I uncheck "Create new revision" on the content type edit page?
The problem was the Content Moderation module (Drupal 8.4).  I guess I have to take the experimental warning seriously.

When Content Moderation module is enabled, you can't change the "create new revision" status for any content type.

However, content types not subject to content moderation will not have the "Create new revision" box checked on the node edit page if the box was not checked before Content Moderation was enabled-- or if the content type was created while Content Moderation was enabled.

This can be confirmed by checking the config for the content type that has been exported by drush, which shows new_revision: false.  This is most likely a bug so I'm off to file an issue.