How to disable TinyMce's Paste plugin from turning image URLs into embedded <img>?

automatically turns pasted image URLs into embedded images

Yes, but only for URLs ending with .gif, .jpg, .jpeg and .png.

And that conversion is one of the "smart paste" features in TinyMCE — another one is when you select a text and paste an absolute URL (that begins with http or https), the text is automatically converted to a hyperlink (clickable link).

The relevant code is below which you can find in wp-includes/js/tinymce/plugins/paste/plugin.js which ships with WordPress — and as of writing, WordPress is still using TinyMCE version 4.9.2.

var insertContent = function (editor, html) {
  if (Settings.isSmartPasteEnabled(editor) === false) {
    pasteHtml(editor, html);
  } else {
    smartInsertContent(editor, html);
  }
};

And if you look at the code, the smart paste features are being applied only if the isSmartPasteEnabled function returns true. The function code:

var isSmartPasteEnabled = function (editor) {
  return editor.getParam('smart_paste', true);
};

So that function basically simply checks whether the editor configuration has the smart_paste option enabled — and it is by default enabled.

Therefore, if you'd like to disable the smart paste features, just set the smart_paste option to false. See example below and try a demo here:

tinymce.init({
  smart_paste: false,
  selector: 'textarea',
  plugins: 'paste'
 });

Disable the smart paste features via wp_editor()

It's very easy. Just set smart_paste to false in the tinymce array:

$content = '<p>Paste an image URL:</p>';
wp_editor( $content, 'editor-id', array(
    'tinymce' => array(
        'smart_paste' => false,
    ),
) );

But remember this..

Setting the smart_paste option to false will disable all smart paste features. If you want to disable just the image URL-to-img-tag conversion, then I think you'd have to copy the Paste plugin and edit where necessary.