Content types already have a disabled property:

{
    "_config_name": "node.type.page",
    "type": "page",
    "name": "Page",
    "description": "Add a page with static content, like the 'About' page.",
    "settings": {
        ...
    },
    "base": "node_content",
    "module": "node",
    "node_preview": "1",
    "help": "",
    "modified": true,
    "disabled": false,  <----- HERE!
    "has_title": true,
    "title_label": "Title",
    "orig_type": "page"
}

But we are not exposing any way to enable/disable content types in the UI. Lets do that

Related article: Drupal 7: How to disable a content type

The database table where content types are stored is node_type. And there's a column disabled in it. We can disable a content type by changing the column value to 1.

GitHub Link: 
https://github.com/backdrop/backdrop-issues/issues/3883
GitHub Issue #: 
3883