Add back test and warning about parent as a string

packages/blocks/src/api/test/registration.js

@@ -749,8 +749,14 @@ describe( 'blocks', () => {
 				title: 'block title',
 				parent: 'core/paragraph',
 			};
-			registerBlockType( 'core/test-block-parent-string', blockType );
-			expect( getBlockType( 'core/test-block-parent-string' ) ).toEqual( {
+			const block = registerBlockType(
+				'core/test-block-parent-string',
+				blockType
+			);
+			expect( console ).toHaveWarnedWith(
+				'Parent must be undefined or an array of strings (block types), but it is a string.'
+			);
+			expect( block ).toEqual( {
 				name: 'core/test-block-parent-string',
 				save: noop,
 				category: 'text',

packages/blocks/src/store/process-block-type.js

@@ -197,22 +197,28 @@ export const processBlockType =
 			return;
 		}
 
-		// Parent being an array hasn't been enforced in the past,
-		// so this is a way to maintain backwards compatibility
-		// with 3rd party blocks that may have been using it as a string.
 		if (
 			typeof settings?.parent === 'string' ||
 			settings?.parent instanceof String
 		) {
 			settings.parent = [ settings.parent ];
+			warning(
+				'Parent must be undefined or an array of strings (block types), but it is a string.'
+			);
+			// Intentionally continue:
+			//
+			// While string values were never supported, they appeared to work with some unintended side-effects
+			// that have been fixed by [#66250](https://github.com/WordPress/gutenberg/pull/66250).
+			//
+			// To be backwards-compatible, this code that automatically migrates strings to arrays.
 		}
 
 		if (
 			! Array.isArray( settings?.parent ) &&
 			settings?.parent !== undefined
 		) {
 			warning(
-				'Block parent must be undefined or an array of block types, but it is ',
+				'Parent must be undefined or an array of block types, but it is ',
 				settings.parent
 			);
 			return;