The confirmation message that Drupal 7.58 shows in English is the following one, where %type is the content type name and @count the number of nodes.
%type is used by @count pieces of content on your site. If you remove %type, you will not be able to edit the %type content and it may not display correctly.
Drupal 7.58 shows that warning, but it lets you delete the content type. What that warning says it's correct: Once the content type used by existing nodes is deleted, those nodes cannot be anymore deleted; they also won't display correctly, since the function that deletes a content type (node_type_delete()) deletes also the fields added to those nodes (with field_attach_delete_bundle()).
In Drupal 7.90, the latest Drupal 7 release available now, if you try to delete a content type for which there are nodes, Drupal won't allow you to delete it. It will tell you you need to first delete all the nodes of that content type.
For example, if I try to delete the Basic Page content type after I created a Basic Page node, instead of the confirmation form, Drupal will show me the following message (although the page title will be Are you sure you want to delete the content type Basic page?).
Basic page is used by 1 piece of content on your site. You cannot remove this content type until you have removed all of the Basic page content.
Although Drupal 7.58 allows you to delete a content type for which there are nodes, it's preferable to first delete the existing nodes of the content type that is going to be deleted, following these steps.
On admin/content, select the content type you want to delete in type
Click on Filter
The list at the bottom of the page will show only the nodes of that content type
Click on the checkbox shown on the right of Title, in the header
Select Delete selected content from Update options (from the list that shows Publish selected content)
Click on the Update button
As side note, Drupal 7.58 isn't anymore secure. The following security issues has been fixed, in releases created after Drupal 7.58. (These are the last five fixed security issues.)
